Since it is not a mod or other external software altering the game externally, but since it still allows you to edit the world generation precisely as you'd like, giving an advantage on people using base world gen, and especially since world gen is no longer part of the run, is it allowed to use advanced world generation for speedruns?

I see that there is no categories for Adventure mode. Why no adding some?

The most obvious to me is Demon% : kill a demon as fast as possible in Adventure mode.

There could be also a Necro% category : read a life and death text and become a necromancer as fast as possible. Or a Slab% category : read a vault slab as fast as possible.

Perhaps a Demonic Slave% category : Have a demon follower as fast as possible, whether it is undead or alive.