Could it be said without using double negative as a sort or exception or not at all?
Same question here, can it be "jsem ničí"?
Possibly. Less likely, but someone might utter it. But I wouldn't accept it.
Jsem ničí vs. nejsem ničí - je v tom vůbec nějaký rozdíl?
To první není moc dobrá čeština.
what would be please the translation of "nejsem nêčí" instead?
wouldn't this one rather mean "I am not all by myself", I am somebody's
No. Please understand that Czech uses negative concord, requires double negatives and they do not cancel each other. It really means that I do not belong to anyone.