Summary

The authoritative short answer to the question as posed is: The chance of William being a pretty decent debater would be extremely high. Born to an English mother, with half of his servants and educators at court also from English provenance, raised to speak a multitude of languages, and directly negotiate himself on a wide range of issues, among them the most detailed provisions before his invitation to the throne, it leaves us not much other choice as the same his main educator made: early on, English was the language he spoke best and Dutch and French needed more effort.

Whatever language the Prince used to speak in these years, I do not dare to say with certainty: probably the most: English, because in 1659 Huygens only found it necessary to maintain this language, whereas Dutch and French had to be taught.
_{From one of his biographers: Nicolas Japikse: "Prins Willem III. De Stadhouder Koning", Meullenhoff: Amsterdam, 1930, p 61, my translation. Link to the Dutch original, "pagina 66".}

→ Since at such a young age William's strongest language was that of his mother: English, and since he continued to receive instruction in it, it is quite unlikely that language alone would have ever been a weakness in his ability "to hold discussions of state (eg audiences) with his ministers in English".

If you are not looking for a detailed explanation, and sources to support that explanation, you can stop reading here.

What follows is long and detailed out of necessity, for corroboration. You have been warned.

The question remains a bit vague in what exactly the criteria are for: "evidence on William's ability (or disability), in the near prelude or aftermath of the Glorious Revolution of 1688, to hold discussions of state (eg audiences) with his ministers in English."

A short answer like the one above met opposition in comments detailing the prior understanding gleaned from general assumptions about European courts and the available information from Wikipedia as counterarguments:

1. comment, upvoted 4 times: No - William was raised in Dutch, by Dutchmen, at a time when the Dutch were in near constant warfare with England and France in alliance. He also courted and married his wife in secret, at a time when every literate person in Western Europe spoke French as a second language. I will need to see hard data to up-vote this claim. ––

2. comment, 1up: As my question makes clear I am looking for details on the ability of William to discuss more complex discussion in English, on matters such as affairs of state, rather than possessing a simple ability to converse. ––

I have found a telling answer buried under reams of irrelevant quotes in your post. If you trim this post down to just an answer to the question posed I will up-vote and accept.

We do not have conclusive evidence that William – perceived as "a Dutchman" took an equivalent test to the one as detailed in Common European Framework of Reference for Languages C-level. We do not have detailed information directly commenting on his command of the English language or how well or not he performed in negotiations conducted in English. But we have quite a lot circumstantial evidence that enables a pretty good educated guess in that direction.

Given the request for "hard evidence" and "detail" surrounded by plausible positions and bits of knowledge from Wikipedia, the following seems to be the case –– and actually needed to answer the question properly:

1 – Looking at the family background of the prince 2 – looking at the languages spoken around him in his youth 3 – looking at his educational background and the fights over it

Since the first comment here needs refutation but continues to get upvotes I am a bit lost how to trim this answer further. A lot of Dutch politicians like DeWitts tried to enforce exactly that education: of Dutch and French like portrayed in comment, but they lost. The "English ties" ensured that William's language was English first and the alliance with France secondary. His Dutch biographies lament as well that we have no direct evidence to directly address the question, so I present the circumstantial points as contradictory and sparse as they are:

Not only was his wife a native English. The languages of the court he came from were French and English and Dutch.

More importantly, his mother was English as well:

Mary, Princess Royal (Mary Henrietta; 4 November 1631 – 24 December 1660) was Princess of Orange and Countess of Nassau by marriage to Prince William II, and co-regent for her son during his minority as Sovereign Prince of Orange from 1651 to 1660.

Together with his diverse education, frequent travels to England, letter writing to English persons, in English, and the amount of political interaction between the two states at large, he must have been fluent, to say the least.

His biographers in Dutch make frequently a point to quote foreign language material in the original language, whether it is in German, French or English.

Given that his personal tutor was the great Constantijn Huygens who was extremely educated and particularly fluent in English, William's own education was diverse and up to certain standards, despite some fights over control and direction of his education:

The letters and memorandums that passed his desk were written not only in Dutch but also in French, English, spanish, italian and Latin. _{Rudolf Dekker: "Family, Culture and Society in the Diary of Constantijn Huygens Jr, Secretary to Stadholder-King William of Orange", Brill: Leiden, Boston, 2013.}

Some letters online, some in English: Briefwisseling van Willem III en van Hans Willem Bentinck (1656-1702)

William was born on November 4th 1650 at 8.30pm. His father William II Prince of Orange died on October 27th in the year 1650 and his mother was Mary Stuart, daughter of Charles I. William was brought up in the Protestant Faith and he spoke English, French, Dutch, German, Latin and Spanish.

So we can see from birth, William III had strong ties to England.

Equally, William hated life in London. He did not fit well in the English court because his plainness stood in sharp contrast to the opulence of the palace; his plain speaking was considered vulgar in comparison with the smooth flattery and hypocritical blandishments of London society; his Calvinism was an abomination to those who, though members of the Anglican Church, possessed no religion at all; and his obvious sincerity could not be tolerated amidst the frivolity of life in the king's palace.

William's mother showed little personal interest in her son. She was sometimes absent for years on end to enjoy the luxuries of the French court and deliberately kept herself apart from Dutch society, affecting not even to understand the Dutch language. His education was first laid in the hands of several Dutch governesses and some of English descent, including Walburg Howard (a stepdaughter of the future Countess of Chesterfield and half-sister of the future 1st Earl of Bellomont);

For the boy Prince, these extraordinary things were the exception. His youth went without shocking events, quietly, in solitude even, several times outside The Hague. His environment was that of his mother, in which the English-style element prevailed. There he must have known the beautiful Anna Hyde, who became the court lady of the Princess-Royale in 1654 and later the mother of his future wife. He did not have an actual governess in the first few years: his mother acted as such. In normal cases this is the best imaginable situation, but one can justifiably doubt whether this mother was able to give her son a proper education. Japikse, see below

And from his biography:

After the financial matters were settled a start was made in 1653 on setting up a household and drawing up an educational programme for the young Prince. The first household consisted of around twenty persons under Mrs Dyck, an Englishwoman. Two years later the Prince was given a permanent governess, Mrs Walburg Howard van den Kerckhoven. She was a daughter of Johan van den Kerckhoven, lord of Heenvliet, and his English wife Mrs Wotton. Heenvliet was Mary Stuart’s chamberlain and one of her confidants. Naturally he had warmly supported Mary’s demand to be sole guardian of her son. Mrs Howard was the widow of Thomas Howard, who had been the Princess’s master of horse.
The author of the Discours sur la nourriture de S.H. Monseigneur le Prince d’Orange cannot be identified with certainty, but he felt that a great future lay in store for the Prince. For this he needed good health, which was to be achieved by bodily exercise. To be able to understand the world around him knowledge of geography and history was indispensable. He should also immerse himself in the study of the art of war. Religion was one of the most important aspects of his education, for the first virtue was to fear God. He must not come into contact with misguided bigotry, and must not concern himself with theological disputes. He must be trained in nobility and magnanimity. The Christian adage, do unto others as you would have them do unto you, was to be held out to him as the rule of life.
He should not meddle in political conflicts but develop self-control by impressing on himself the motto that he who is master of himself is master of all others. Finally he must show respect to the States of Holland and thus subordinate his own interests to those of the Republic.
William III did in fact acquire the necessary self-control, which was to become second nature to him, and only rarely lost it. After a sporadic outburst his natural reserve immediately reasserted itself. He was always to show respect for the proponents of the True Freedom, but it must certainly have cost him some trouble, especially after he found out what a trick the States of Holland had played on him a year after this educational programme was drawn up.

He was also afraid that the Prince’s pro-English entourage would call in the aid of Charles II against his own rule. A letter from William III to his English uncle of 2 February 1661 shows that this was already happening. In that letter, clearly inspired by his entourage, the Prince wrote to Charles urging him to press the ambassadors from the Republic to designate him for the positions held by his forefathers. Orange felt that ‘there will never be a better opportunity to secure that designation’.
_{Wout Troost: "William III, the Stadholder-King. A Political Biography". Routledge: London, New York, 2005.}

He was quite a talent. He was writing letters in foreign languages and negotiated personally political affairs with native speakers of other languages about intricate affairs, also of state. It seems that the usual multilingualism of much of the nobility of Europe at the time came in quite handy, later. But for him, not every language was that easy from the start:

Het onderwijs in schrijven, lezen en rekenen begon later dan dat in den godsdienst. Het werd eerst alleen gegeven in den tijd, dat de Prins te Breda vertoefde, dus ongeregeld, en wel door Abram de Raguineau, voorlezer in de Fransche kerk aldaar. Hoogstwaarschijnlijk heeft Willem, voordat deze lessen een aanvang namen, reeds schrijven en lezen geleerd: in 1656.
wilde men hem ten minste een brief laten schrijven, maar zijn moeder vond beter, dat Trigland het voor hem deed. Welke taal de Prins in deze jaren gewoonlijk- sprak, durf ik niet met zekerheid zeggen: waarschijnlijk het meest Engelsch, want in 1659 vond Huygens het alleen noodig deze taal te onderhouden, terwij Nederlandsch en Fransch bepaald geleerd moesten worden.
____The teaching of writing, reading and arithmetic began later than that of religion. It was first only given in the time that the Prince lived in Breda, and so was unregulated, by Abram de Raguineau, reader in the French church there. Most probably William, before these lessons began, already learned to write and read: in 1656. They wanted him to write at least a letter, but his mother thought it better that Trigland did it for him. Whatever language the Prince used to speak in these years, I do not dare to say with certainty: probably the most: English, because in 1659 Huygens only found it necessary to maintain this language, whereas Dutch and French had to be taught.
--> Source: Japikse, see below, p61.

[…] Chapuzeau was a failure: he disappeared after his bankruptcy in February 1661 and his influence on the Prince could not have been significant. No reference is made to a successor. Maybe in these circumstances lies the explanation for the fact that the Prince later wrote such miserable French.

The French envoy tells us that Mary Killigrew also controlled her husband and the Prince, and an English trait in the defense is also observed from others, not unjustly, as will soon be apparent. However - how Zuylesteyn carried out his part of the upbringing, how Bornius and Raguineau and perhaps others have theirs, our reports are silent about that.
--> Source: Japikse, see below.

An old assessment clouds language ability with court expectations:

As Lord Macaulay in his History of England observed: “He was in truth far better qualified to save a nation than to adorn a court… . He seldom came forth from his closet, and when he appeared in the public rooms, he stood among the crowd of courtiers and ladies, stern and abstracted, making no jest, and smiling at none. His freezing look, his silence, the dry and concise answers which he uttered when he could keep silence no longer, disgusted noblemen and gentlemen who had been accustomed to be slapped on the back by their royal masters…. He spoke our language, but not well. His accent was foreign: his diction was inelegant; and his vocabulary seems to have been no larger than was necessary for the transaction of business.

Nonetheless, William was understood when he spoke English, but then he was not perfect in every way, and some tried to emphasise their grievances:

Hij sprak het Engelsch met een vreemd accent, wat bij toespraken tot het Parlement bizonder opviel. Zelfs kon hij 't in Whitehall heelemaal niet uithouden; zoo spoedig mogelijk vertrok hij naar Hamptoncourt, al was dit in slechten staat: daar kon hij ten minste jagen!
____He spoke English with a strange accent, which was particularly noticeable in speeches to Parliament. He even could not stand it in Whitehall at all; as soon as possible he left for Hamptoncourt, although this was in bad condition: he could at least hunt there! (p281)

Source: Nicolas Japikse: "Prins Willem III. De Stadhouder Koning", Meullenhoff: Amsterdam, 1930. Online, PDF in Dutch.

In conclusion: his spoken language abilities in general and his command of English in particular – in all probability – did not impede his conversations with his servants, secretaries, councils or ministers. His personality might be a much bigger hinderance. Despite his great diplomacy skills he also displayed an astonishing lack of English court etiquette, for traditions and some intricacies of the English politics.