1. Thrall is Master level, but does that mean stronger than Dread Zombie (Expert level), or merely different? Usually descriptions state "weak", "powerful", "even more powerful", but not here.
Thrall is permanent, but only affects people, and only up to lvl40. Would Dread Zombie be better, just not permanent zombie?

2. Can these spells be acquired via looted books (random drop), or only from Phinis Gestor? Due to RP reasons I don't want to join Winter magic school, so the only other choice is console.

Dread Zombie only affects creatures/people up to level 30. Dead Thrall also doesn't disintegrate your minions when they die, so as long as the corpse is accessible they can be raised repeatedly (though the spell is very costly and slow for combat). At least without using some kind of PC mods (xbox gamer, I'm pretty ignorant of that sort of thing), Dead Thrall can only be acquired from Phinis after completing the conjuration quest. I suspect he is the only one to sell Dread Zombie as well, but I'd be willing to bet it could be found as loot (very rare though) as I've found expert level spells randomly on occasion.