Is this talk just a convention?

Conventionally, this heap of bones types an answer to that heap of bones, I think. But that may just be my conceit.

The last two lines of SN 1.25 for example,

Skillful, understanding conventional usage,
they use these terms as no more than expressions.”

Loke samaññaṃ kusalo viditvā;
Vohāramattena so vohareyyā”ti.

I think the word "conventional" translated there is loka which means "world".

That word is also used in phrases like "loka-dhamma" (affairs or phenomena of the world, e.g. "wealth" etc.); and some schools mention "conventional truth", etc.

I think that the kind of speech you question (i.e. "I have craving for music. Do you have craving for ice-cream?") certainly does sound 'worldly', or conventional.

I suppose an important question is whether that speech is also "skilful" or kusala -- and unfortunately maybe it isn't.

The answer seems to be that Buddha used 'I' for sake of convention.

The Buddha doesn't use personal pronouns for "the sake of" convention. He uses them because they are useful in communicating with others. That's why we all use them. It is not like he only does so because others are deluded. Again, there is nothing whatsoever wrong, mistaken, or incorrect with using personal pronouns. Using personal pronouns does not contradict the absence of identity-view at all.

Given Anatta , is the following talk a mere convention?

All talk is convention. Or better put, all talk takes place via the rules of mere convention.

I have craving for music. Do you have craving for ice-cream?

If someone says this they are using words - which are governed by the rules of mere convention - to relay an idea of something to someone else. In this case, they are relaying that they feel a strong desire or attachment for music and are asking whether another has a strong desire or attachment for ice-cream.

Is the 'I'and 'you' used in the talk above merely a convention or is it real?

Absolutely everything in the above talk is mere convention. That is, "I" and "you" and "ice-cream" and "music" and "craving" and "having" are all mere convention and are not the slightest bit real. However, only enlightened Buddhas and Arya beings (in meditative equipoise on emptiness) directly perceive the unreality of all the above. For the rest of us, we are usually fooled in believing the above are real. Some of us have doubts about whether they are real and some of us are tending to believe (without being thoroughly convinced) that they are not real. Understanding emptiness at a conceptual level or with reason is when we incontrovertibly know that nothing is real.

What I am describing for you is the Middle Way. Saying that clearly existing things - like the self - don't exist at all is annihilationism and is incorrect. Saying that things such as the self truly exist is eternalism and is incorrect. Instead, there is a subtle middle that is hard to see. It is the in-between of existing without falling to the extremes of annihiliationism or eternalism. It is hard to see or understand, but that is how things actually exist: in the in-between of those two extremes. This is the Middle Way.

As ruben2020 pointed out so well in this previous answer the Middle Way was taught by the Buddha even in the Pali-canon. See this sutta:

"'The one who acts is the one who experiences [the result of the act]' amounts to the eternalist statement, 'Existing from the very beginning, stress is self-made.' 'The one who acts is someone other than the one who experiences'[2] amounts to the annihilationist statement, 'For one existing harassed by feeling, stress is other-made.' Avoiding these two extremes, the Tathagata teaches the Dhamma via the middle: ..."

The Mahayana expounds upon this Middle Way by teachers such as Nagarjuna who gave a full and detailed account of how the Middle Way works in his Fundamental Treatise:

"Nagarjuna's influential Mūlamadhyamakakārikā deconstructs the usage of terms describing reality, leading to the insight into śūnyatā "emptiness". It contains only one reference to a sutta, the Kaccāyanagotta Sutta from the Samyutta Nikaya:

"Everything exists": That is one extreme. "Everything doesn't exist": That is a second extreme. Avoiding these two extremes, The Tathagata teaches the Dhamma via the middle."

If I tell you: "Your future is in your hands."

Would you look at your physical hands and reply, "but where is it? I can't see it!" I don't think so.

"In your hands" is a mere conventional figure of speech, not to be taken literally.

Similarly, the terms "I", "my", "mine", "you", "your" etc. are simply conventional terms when used verbally by the enlightened ones. They don't literally think that "I" refers to their body or consciousness or mental fabrications or feeling or perception.

It's all about mental context, my friend. If in your mind there's a (silent) conviction that you see, you act, you were born, you will die, this is you etc. - if your subjective experience is like that - then for you there is Self. Even if you don't use the word "I", there's is still belief in I in your mind.

If in your mind there's a silent understanding of how things work, how mind works, and how illusion of I is generated - then for you there's no "Self" or "No-Self" - you clearly see how it really is. Even if you use the word "I", there's no belief in I in your mind.

All talk is conventional, because "conventional" means we assume we are more or less in agreement what each word means. What matters is your mental context.