[Kobal2]

WAG: the reason it's 12 instead of the more intuitive 10 is that with only 10, on average theses spells wouldn't work on a regular human (46% chance to win an even roll), which is quite crappy. Who would cast a spell that is more likely than not to do absolutely nothing ?
With 12 however, spells works 62% of the time, making them worth your while to cast.

As for the second part of the question : instant death spells will just remove all of the victim's HP, trigger the Phoenix Pyre explosion and the target will respawn somewhere else with full HP, some fatigue and most probably an affliction. Phoenyx Pyre only stops working when the target has 100+ fatigue.