demons appear, the scared human asks "what are you ?" and the demons
reply "demons for some, angels for others".

Suppose I write the above in klingon:

nargh qa'pu' Hurgh, 'ej ghel Human'e' lughIjbogh: nuq tlhIH.
jang qa'pu' Hurgh:
'opvaD qa'pu' Hurgh, latlhpu'vaD angels

If you're calling demons qa' Hurgh, why aren't angels qa' wov?

1. at the {ghel Human'e' lughIjbogh} I think the {-'e'} isn't
necessary. Am I correct ?

Yes, it is not required or, in this case, even helpful. We do not believe that an elided noun or pronoun can be the head noun of a relative clause. (Exception: a sound file on the Star Trek: Klingon game has Okrand saying Dajatlhbogh vIyajbe', which seems to do exactly this. The sound file was not used in the language lab, so we don't know if it's meant to be a complete sentence.)

2. at the {'opvaD qa'pu' Hurgh} is the {'opvaD} on its own acceptable,
or is it absolutely necessary to write something like {'op HumanvaD}

Acceptable. I don't see any reason why "some what?" wouldn't be obvious.

3. and most important: since at the above story the context is clear,
is it correct for the demons' reply to be just {'op HumanvaD qa'pu'
Hurgh} "demons for some", or is it necessary to write {'op HumanvaD
qa'pu' Hurgh maH} ?

In a formal grammatical sentence, you need a verb. If I understood you to be using grammatical shortcuts, I'd be lenient.

4. If I remember correctly, some time ago we had said that
{'oppu}/{'opmey} is wrong, right ?

I don't know if they're actually wrong, but they don't make a lot of sense to me.

-- 
SuStel
http://trimboli.name