I fell through a hole covered by loot, and left my pet Master Lich long enough for it to go PEACEFUL. Normally, it wouldn't be a problem to drain its level and retame, But, this time I equipped it with GDSM and a shield of reflection. Will this work through that much MR, or should I just kill it to prevent future problems? And, what about Cancellation?
The answer to the title question is "No", however there's a lot of special cases involved here.
Will monsters benefit from gear that gives magic resistance or reflection?
Yes. As long as the monster is capable of wearing the armor, they will gain magic resistance/reflection. This will protect them from death rays, teleport traps (MR), ray attacks (reflection), just like with a player.
Does equipment with magic resistance boost a monster's innate magic resistance?
No. Monster-style magic resistance is completed unrelated to player-style magic resistance. The two protect against different things, and do not augment each other.
Will MR or Reflection protect against either drain life or taming spells?
No. Drain life is a beam and bypasses reflection. Neither MR or reflection will protect you or any monsters from drain life attacks, and will not protect monsters from taming.
Can I drain life a Master Lich?
No. Liches are undead and are immune to drain life.
Can I re-tame a peaceful Master Lich?
No. Master and Arch liches are covetous, and cannot be tamed. Unlike other covetous monsters, these liches are not unique, and their lesser forms are not covetous, so it is possible to end up with a tame covetous lich by polymorph or by leveling. But a lich that is already Master/Arch cannot be "demoted" and cannot ever be tamed.
Can I cancel a Master Lich?
Not easily. Master and Arch liches have a (monster-style) MR value of 90. Master liches will be immune to wands of cancellation at level 22, and even at the lowest level for a Master lich (17), it will resist the wand 93% of the time. A high level spellcaster can cancel a lich, but it will probably still take a few tries.
Incidentally, cancellation will prevent it from casting spells if it becomes hostile, but will not prevent its covetous warping behavior.