On point #2 if the landing gear survived normal landings but came off on a dead stick then your friend needs to correct his dead stick landing technique.

#3 gas engines do vibrate more than glow, maybe it can stand to be reinforced a little or maybe it just didn't get glued properly, your call here. I don't mind beefing up this area because most all of my planes have needed extra weight up front to balance anyway so why not make it useful weight?

#5 I have a 1/4 scale Tiger Moth, it weights about 18 lbs, the bottom is just a few stringers and a little plywood and I transport it in my car with the seat back.  No issues.  And your plane looks like it is at least as rugged if not more so in some areas.

There is a reason a 3D plane is built light, if it doesn't look strong enough for you then maybe you need to practice with a plane that is built heavier for someone who is learning 3D, there are 3D planes that are built on the heavy side.