I think in full-full version of Hatgame all projectiles will be in 3D.
(then I will also fix the movement from the walls in the full version)
Also, what did you mean by difficult to define in 3D space?
Here is an example of what I'm talking about:
This is your projectile. The square is your 2D sprite. The wireframe is your collider (invisible in game).
As you can probably see (or not see) the side view makes the object completely invisible to the renderer while the collider still has full volume. This means that the player can get hit by invisible things. I hope this makes the problem more clear!