RE: CRRC PRO GF26I
If the spinner back plate is just touching it may not be enough to get a good bite on the hub serations. Here is something to try to keep the propshaft from getting spit back at you when it pops backwards.
Unscrew the propshaft, take the lock washer out, clean the threads and put red Locktite (262) on the threads inside the propshaft and reinstall without the lockwasher, let it sit overnight. This will let the spinner back plate make full contact with the prop hub so the serations can do their job.
As far as starting backwards, this is the first 26cc engine I've heard of doing it. It was more common with the 50cc and was always associated with the timing being advanced to far. Check the timing with a degree wheel and see what it is exactly. They are usually at about 30 degrees from the factory but I have seen them as far as 36. Set it at 28 degrees and give it a try.
Besides that you can try to flip it over either a little harder or a little softer, sounds weird but sometimes it works. Get the engine right itself though first, if you don't, there will always be something else going on that will be a pain to deal with.
If you can't figure it out don't give up on it, take a break and fly something else, send it in and I'll take care of it.