This may answer your gravity angle question. In short, simply set gravity to 0 and add constant force that represents the gravity - at desired angle.
As for sprites behaving as if they were within solids - well, you've got to make sure the transparent background is not part of the physics world. So the background object shouldn't have physics behavior nor solid/jumpthru attributes; in that case it will be ignored. For collidable structures use separate objects.
Or so I believe, at least.