How alcoholic were ancient Greek and Roman wines, before and after dilution?

That is very difficult to put a single number on.

Like today, ethanol content in wines ranges from 5–25%, but usually between 9–16%.

It depends a bit on how strong one prefers, or how the Greeks preferred the ethanol content to be.

Traditionally it was that one consumption unit 'equals' – or let's say 'corresponds' – to 10g pure ethanol. (One shot of schnaps, one beer, one glass of wine.) But the Greeks went by taste and aroma more than anything else, despite still measuring the virtue of moderation by the amount of cups consumed.

Then there is this cultural thing of Greeks thinking low of anyone drinking his wine undiluted. As a drunkard or guilty of barbarism. That does of course not exclude people like Alexander the Great and his philoi friends doing just that.

For how much the wines were diluted we can only turn to the textual evidence.

Aristophanes for example gives in Plutus an allusion to the following advice for getting a nice wine fit for a symposion:

• (too) strong mix: half wine, half water
• nice mix: two parts of wine, three parts of water

But a point blank quote might satisfy your curiosity:

The uses of wine
Wine had many uses for the Greeks. It was of course important as a food and drink (it was doubtless often safer than water), and the symposium, which centred around the drinking of wine, was one of the most important Greek social forms.

Wine was almost always drunk diluted with water:

the ratio varied, normally ranging between 2 : 3 and 1 : 3, which would give a range in alcoholic strength of about 3 to 6% and generally at the lower end of this range (roughly the same as British draught beer).

Weaker mixtures are disparaged in comedy (and even 1 : 3 called for a good wine),

but 1 : 1 was considered by some dangerous to the health, and the regular drinking of unmixed wine, a habit confined to barbarians, was believed by some Spartans to have caused the insanity and death of their King Cleomenes. The mixed wine was also normally cooled, sometimes in special pottery coolers; the very rich added snow.
_{— Jancis Robinson & Julia Harding: "The Oxford Companion to Wine", Oxford University Press, 2015.}

But that is just a very rough estimate for the history of Roman and Greek wine.

While Pliny mentions a ratio of 1 : 8 is found (Natural History 14,6,54), in Homer's Odyssey even a ration of 1 part wine to 20 parts water is mentioned:

And he had given it me because we had protected him with his child and wife [200] out of reverence; for he dwelt in a wooded grove of Phoebus Apollo. And he gave me splendid gifts: of well-wrought gold he gave me seven talents, and he gave me a mixing-bowl all of silver; and besides these, wine, wherewith he filled twelve jars in all, [205] wine sweet and unmixed, a drink divine. Not one of his slaves nor of the maids in his halls knew thereof, but himself and his dear wife, and one house-dame only. And as often as they drank that honey-sweet red wine he would fill one cup and pour it into twenty measures of water, [210] and a smell would rise from the mixing-bowl marvellously sweet; then verily would one not choose to hold back. — (Odyssey IX, 208)

Note that such dilutions are most probably the result of using a very aged wine. Since they weren't sealed as good as now common, they became quite syrupy. But apparently not bad at all, to ancient palates.

Everyone seems to have agreed on which these were, and at the pinnacle of repute were wines that came from the slopes of Mount Falernus, to the north of Naples. Made from the Aminean grape, these golden or amber-colored wines were probably high in alcoholic content, since Pliny the Elder recorded that they might “take light” when a ame was applied to them. The most fabled Falernian vintage was harvested in 121 b.c.e. Not only was it widely praised at the time, it was served to Julius Caesar a hundred years later, presumably to his entire satisfaction because someone was apparently brave enough to offer it again to Caligula in 39 c.e., when it was 160 years old.
_{— Ian Tattersall & Rob Desalle: "A Natural History of Wine", Yale University Press: New Haven, London, 2015.}

More on the archaelogical side of things:
— Patrick E. McGovern, Stuart J. Fleming & Solomon H. Katz: "The Origins and Ancient History of Wine", Food and Nutrition in History and Anthropology, Routledge, London, New York, 1996.

Accurately analysing archaeological finds towards their ethanol content is usually impossible. Empty vessels are sometimes analysed to ascertain their overall content. But proxy substances like tartaric acid can only be used to identify not measure the liquids. The presumably oldest liquid wine bottle from 300 AD remains unopened and is thought to have lost all its ethanol anyway by now.