When used as the adverb "merely", neH should follow the verb. Appearing at the end of the sentence like that, it looks like the verb neH which would mean, "They want to live twice." (Which I suppose could be what you meant, but I thought it was worth mentioning since either was a possability.)
It didn't come out exactly as you wanted it, but I think you did great! You are here to learn and you accomplished that. I would like to encourage you to make more mistakes! You can only succeed if you try and you can only improve if you fail. Trying and failing is a fine thing and I commend you!
This is a bit tricky. cha'logh yIn chaH neH means They alone live twice. cha'logh yIn neH chaH means They merely live twice (that's it, they just live twice, they don't do anything else of note). Neither is really what you want. It's unclear what cha'logh neH yIn chaH would mean (they live a mere two times? they live two times that are isolated from other times?). We don't have any rules for adding the adverbial neH after a sort-of-adverbial repetition number. My hunch is that this last form would work, but I have no evidence for that.
It's not untranslatable; it's just that any translation is going to have a subtly different meaning, or is going to require rewording.
You could, for instance, say cha'logh yIn chaH. latlh yIn Suqbe'. They live twice. They do not obtain more lives. It expresses the same thing as They only live twice, but it needed to be worded differently and is no longer pithy.
I don't think cha' yIn neH yInlu' works either. It says one lives two lives and not something else: one does not live elephants or spaceships, for example.
The phrase is supposed to mean "two, not more." It's the "two" or "twice" that needs to be modified, and when numbers are used adverbially or to modify nouns, it's not at all clear that neH can be used on them or what it would mean.
Maybe it is acceptable to use neH on an adverb the way you would use it on a noun. Or it might even be possible for it to act the way you want when following the verb. We don't have any clear support for those interpretations, but nor can we say those are definitely not possible. However, you could probably say, cha' yIn neH yInlu' ("One lives only 2 lives.")