Do I need to protect my bike from jostling around if I put it under the bus?
With other items sliding around I'd suggest a case where you don't need to take the bike fully apart to pack it. I think there are some you can just take the front wheel off of and pack it. If you can't do something like that you could use bungie cords to secure it and go to the hardware store and get some foam padding to put around the frame and drivetrain. It's not 100% but it'll manage the risk a bit more. I'd be less concerned about the frame snapping or bending and more about derailleurs breaking or spokes breaking.