I do agree that bush is a "broken" spell that feels trivially easy. Maybe that could be justified as a way to give beginners an easy way to get a clear if it were slightly easier to reach, but people have said that the bush orb was too hard to get. Maybe I'll add a hint about that. Good point about the fog of war as well; I might add in a "disable fog" option in gameplay settings.
As for your questions:
1. how do you design this type of multipurpose level?
Mostly, lots of iteration, i.e. repeatedly changing and re-designing things until it's in a place I'm happy with. It's mostly a matter of putting in a lot of time, but it's also about having a lot of goals/constraints while also being willing to try and re-work a lot of things.
2. did you design the spells around the level or vice-versa?
It's hard to answer this question. I think a lot more tweaking/iteration went into the level design than the spells, so I guess it's a case of spells influencing the level? There are much fewer "levers to pull" (i.e. stuff that can be changed) in the spells than the level, so it makes sense to have the level adapt to the spells.
3. are all of the trophies intentional, or did some just happen to be possible so you included them?
Mostly, I just designed the level and figured out what trophies were possible. I suppose that the trophies also influenced the level design, especially once the set of "minimal trophies" were locked in.