Wrt MH issues: none of us here know anything about anybody (profound statement of the say - you are very welcome).
IMO and IME serious MH illness is really quite recognisable, more low level Mental 'Unwellbeing' is much harder to spot.
People with MH difficulties also watch TV, read magazines, speak to their neighbours, interact in all sorts of ways with the world around them and are all around us (or ARE us).
Trolls are at best harmless fun-seakers and at worst nasty bastards, whereas most posters are not looking to upset anybody it is easy to write a title or an OP that is 'triggering' for various people for various reasons. Nasty trolls are not 'mentally ill', they may be simply thoughtless or enjoy hurting people which is likely due to a personality disorder and not illness. They know fine well what they are doing - pricks!!
I don't think it is possible to interact with the world and not potentially get hurt, particularly when vulnerable.
I do not see it as the responsibility of a forum to be any more protective or nurturing than any other gathering of people. So, just as it is not acceptable to throw a punch in the pub or start shouting abuse at somebody in RL, it is not acceptable here.
I think it is our responsibility as users of this forum to bee-hive, but equally I get very impatient when one cannot mention 'loofahs' (WTAF?!) without a trigger warning and I am saying that as somebody who lives with somebody with an 8 legged beastie phobia and experience of other phobias.