From what Ive read melee 'spells' use weapon crit, ranged 'spells' use spell crit.
This is not correct. Melee versus ranged doesn't determine the method. I will say that most of the abilities use weapon crit. It was described in another thread by saying that if it looks like you're attacking with a summoned weapon than it uses weapon crit, like Impale. Spells that actually look like spells use spell crit, like Strife.
Veiled Strike, Teleport Strike,Death Stroke and their morphs -Crit Chance is given by weapon crit. The damage increases with magicka and they are mitigated by armor.That means two consecutive non critical Surprise attacks do slightly different damage due to armor reduction.
Assassins Blade and morphs did not appear to ever crit when I tested it. Doesn't really need to though.