Duo has trained me to translate "xin lỗi" as "sorry" just to have my answers accepted. In real life l say, "Excuse me" most of the time to be polite and say, "Sorry," only apologetically. I guess I use "Pardon me" and "Forgive me" similarly but not as often. This is ideolect, of course; not grammar.
It accepts "excuse me" as well. That's what i wrote each time, because that's what I'd say, and it has accepted it each time. "Sorry" doesnt really make sense in this situation.
I'm afraid to try, but if it doesn't accept "some bread," it should, because that is how we would say it. It's most peculiar to say "I want bread," and indeed it would give away that you're a non-native speaker by saying that. Yes, there are isolated instances when it might be said, but by and large it would either be that, this or some bread, and usually it would be some.
My understanding is that xin lỗi is used as "I'm sorry" and as "excuse me" ... depending on the sitiation. Xin hoi might be used like excuse me when you want to get someone's attention, like a shokeeper, waiter, etc...