I used 5 flat damage as an absurdist example that even at that much damage, it wouldn't be a "good" strategy to use poison. Five flat damage would most certainly be broken early game and in the arena.
Ten percent damage would make poison a better status than it is now late game, but still very weak overall because of the general pacing of the game past very early game. Do you really want to poison someone and allow a bunch of turns to pass so you can reduce the number of true damage casts you need to kill them by one? It certainly wouldn't be faster, but if it could be classed as "more fun", that is also valid.
Poison just has very poor synergy overall compared to stuff like frozen, burning, even silence and death mark. Sure, poison doesn't auto cleanse, but battles don't last long enough for the other stuff to auto cleanse most of the time anyways.
Yes. At least, if you can get the 600+ gold keys from a level 6 brown task by your second week playing. Having access to just most the rares and commons gives you stuff like alchemist and ranger, which you can use to set up and team wipe the opposing team with very little level investment or back and forth. Things like Chimera can pack a punch in this manner, being that they are low mana cost for the burn and poison as well as taking most the total hits of the strongest thing on their team, and it can most definitely get this way if you don't have access to any mana converters, but on my new account I've never been threatened by just poison, simply because of the pace of the game.