I think you are almost certainly technically correct, although a majority of people would probably say it with "us" at the end. (Similarly, most people would say "than him" or "than her" rather than saying "than he" or "than she," even though the latter may be prescribed as the correct English constructions.)
I would say not.
You can say "They eat less than we eat", but if you don't want to repeat the verb, you cannot imagine it to be placed automatically, because the pronoun (we/us) is now the object of a preposition phrase.
(EDIT) I am convinced that the 'we'-version is also correct (in fact originally the only correct one, similar to German) and just like many other things in English, it has deteriorated.
Both of the translations you suggest are correct. One can also omit the second verb: "They eat less than we." This is also correct, but will sound a bit too formal to most people.
In informal speech, native speakers (especially young people) would more likely say, "They eat less than us." Many people claim that this is incorrect. But the Merriam-Webster Dictionary disagrees. Along with Shakespeare, Faulkner, and T.S. Elliot, the editors at M.W. are comfortable with "than me," "than him," "than us," "than them," and "than whom."
In short, according to the experts, "than" can be used as a conjunction or a preposition. So both "They eat less than we," and "They eat less than us" are grammatically correct.
@ maquignon: In the sentence in question and in many like it, "than" can be used as either a preposition or a conjunction. (You are right that it cannot be both at the same time.) That is the consesus among experts:
In simple comparisons in contemporary English, than often takes an oblique pronoun, which lexicographers and usage commentators regard as prepositional use and as standard. ----Wikipedia:
- They eat less than us.
- They eat less than we.
- They eat less than we do.
- They eat less than we eat.
All of these are standard English. One of them uses "than" as a preposition. It is structurally and semantically similar to this:
- They eat less compared to us.