One of the things I think people get wrong about the Solozzo-Michael sit down is that they suddenly start speaking English, In the presence of McCluskey, no less.

I believe this is wrong. Sure, we the viewers see this happen but it's just a device used by the director because this is the section of the conversation, indeed the only section of the conversation that he wants us to hear directly.

What do we see/hear?
Michael's imploring Solozzo: "What I want -- what's most important to me -- is that I have a guarantee: No more attempts on my father's life." Remember how he struggles to find the words, just as he did in the Italian section.

Then there's Solozzo's reply: "What guarantees could I give you ... You think too much of me, kid -- I'm not that clever ... " Especially the smug, self-satisfied way Solozzo delivers his reply. He really thinks Mike is a weakling and that he is going to crush him and his family. Of course we know exactly what is going to happen, and it just makes Solozzo's comeuppance that much more satisfying.

Also, I believe that if Michael had actually started speaking English Solozzo would have shut him down in an instant. Because of McCluskey. Sure, they're allies now but who's to say their relationship wouldn't sour in the future. (I realise this is moot given the outcome) It was Solozzo who decided they would speak Italian in the first place and it would be he who would decide otherwise.

This is a fairly common film device used by directors to show that the character is still speaking his native language, but the switch to English is simply a viewer aid. Two examples of major films involve Sean Connery in The Hunt for Red October, and Tom Cruise in Valkyrie.