"they will eat more than we". reported. http://www.grammar-monster.com/lessons/than_I_me_than_he_him.htm
Theoretically correct, but nobody speaks like that unless they want to sound like a pedant. Advice from an English teacher - in normal conversation "more than us" is fine, and there is no grammatical reason why it shouldn't be.
If you're worried what the (misinformed) pedants might think, say "more than we do", but no normal mortal says "more than we."
Indeed. In English "we" and "us" are often interchangeable in such sentences and it can lead to ambiguity. In Polish if you change "my" to "nas", it will mean that they will eat not only us but also something more.