That's a lot of questions in one post. I'll address the Diamond perks parts...
I would go out of your way to say you DO NOT want this soda or water to count as a D drink and ask to be charged just to be safe. Conversely, I always say "as one of my Diamond drinks" after ordering one.
They do not roll over and I would never consider trying to retroactively use them like you mentioned. Just sounds like a giant headache.
Yes, the day starts at midnight. You actually get 4 more drinks on disembarkation morning and can order mimosas or whatever with them.
Yes there are still snacks in the DL as well as the coffee machine for cappuccino, espresso, latte, etc. There is also the breakfast spread just like pre-covid.
Hope this helps.