If I weaken a legendary enemy on normal, then right before I kill it switch to survival; will I get a better legend drop?
I haven't confirmed this through testing, but it's my understanding that enemy drops are determined at the time that the enemy is spawned. Therefore, changing your difficulty after the enemy spawns shouldn't affect the drops.
This article on farming legendary drops indicates that the drops are set once the dungeon spawns. This leads to the same conclusion.
Loot is normally determined by a LeveledList which is a placeholder for items.
A LeveledList will be resolved on spawning an Actor/Container replacing the List with item calculated by Level, difficulty and a random factor.
Also a farming technique with spawning enemies in survival and kill in a lower difficulty will work one time without mods, as the engine limits to only one downscale from survival and then you stay below survival.