Ok, so after having take a look at your .capx, I see what you mean by "bullet overrides solid behavior" but I can't think of a correct solution right now. I think though, your idea of handling its movement with the "IsColliding" variable is correct, and that you should keep working in that sense. Also, I guess bullet behavior is not the best option, you can do the same thing it does using the action Sprite.Move forward (in Size & position) once you put the angle to player's position.
From there, in your event "mushy_env on collision with enviro" you could manually change its position alongside enviro.
As I said though, I don't know if that's a correct solution.
Considering your question about group objects, are you talking about families ? (another tutorial about families)
"Families" only are implemented in CC atm, not in C2 (yet).