If we can say:

vIghro' tIQ wIjon 'e' neH DaSaH
you only care that we capture the ancient cat

If neH is okay after an 'e', then this means You care that it is only the case that we capture the ancient cat. neH after a pronoun is the same as neH after a noun, and neH after a noun means only this, not that. Your English translation, on the other hand, invokes the after-a-verb sense of merely: You care that it is merely the case that we capture the ancient cat; You care about the triviality that is our capturing the ancient cat.

So if it's a legal sentence, it doesn't mean what you think it means.

If you want to express that I only care about capturing the ancient cat and not something else, you'd need to say something like vIghro' tIQ wIjon 'e' DaSaH; latlh DaSaHbe'.

Then why couldn't we say as well:

vIghro' tIQ wIjon neH DaneH
you only want that we capture the ancient cat

Same problem as above. In addition, the neH here would almost certainly be interpreted as trivializing the jon instead of making exclusive the sentence vIghro' tIQ wIjon.

-- 
SuStel
http://trimboli.name