i dont think wtch is considered a dot, hard to be sure about any mechanics in this game but it would seem to simply be a repetition of the hit an enemy receives.

in what way are you wondering if insidious effects wtch? if the -30% non dot effects it? again who knows for sure with wolcen but afaik yes it would effect the damage dealt by it.

this is all afaik stuff, ive looked a lot of mechanics discussions etc and tried a bunch of stuff out, this isnt gospel but this is just what im presuming atm. that -30% damage is additive with all your increased damage, so u probably got like 700%-800% increased damage from passives, gear and attributes, so if u have 700% you now have 670%, its not really a big deal, its not literally cutting off 30% of your total damage. twch essentially repeats a direct hit again after 1.5 seconds. it doesnt repeat the exact same damage dealt, it basically hits the monster with the skill a second time, so if the skill deals 800-1000 damage and you hit the monster for 880 damage twch will hit it again and might roll 820 damage, or 990, it seems to be an entirely new hit. this second hit cannot crit and it probably cannot inflict ailments, also ailments are not hits so it wont repeat ailment damage, just direct hit damage from skills.

this is why i would presume its not considered a dot in any mechanical sense, because it seems to be a second direct hit in all observable ways.