I've nursed all of my kids until they weaned on their own (around 4yo). Well, except my last, who's almost two--he's still nursing.
I've always multi-tasked, and I find that nursing makes life SO much easier. Plus, the extra insurance against illness is huge. I, too, drink a decent amount of coffee. I also frequently drink a Mike's Hard Lemonade with dinner. I don't think moderate drinking is an issue, especially with an older child who's getting some solids. I'm a very moderate drinker, hate feeling "drunk", so I don't worry about that, for me, anyway.
Congrats on nursing your little guy for as long as you have! To answer your other questions: BM is high in nutrients and calories, so I'd say that your son is probably getting what he needs. If he weans, you'll have to be a lot more vigilant about food choices. You know, this may sound weird, but I'd let your daughter try nursing the next time she asks. I doubt that after all this time she'll remember how, and that way she maybe would feel validated?
As far as sleep, not sure. I find that it is so much easier to get a child back to sleep by nursing, so I'm in favor of not night weaning either unless it is child-led.
Good luck mama!