Would gutting the spellbook of its pages count as "destroying" it
Yes, and it also wouldn't interfere in any way with transcribing the spells. I'd go for some other idea.

For traps, the problem with Maze is that the creature is released unharmed after not too many rounds. The spell's main utility is in breaking up groups of enemies, so you can deal with a few at a time, but if you don't have anything else challenging your enemies at the same time, it's worthless.

And with Gate, yes, the demon will be angry, which might mean that it decides that, since you got it stuck on the Material Plane, it might just destroy your phylactery in revenge. Bad idea.