Yeah, I adjusted mine multiple times without any luck. I had extended the shifter rod, as mentioned on this board, as well as some videos I watched, and that resolved the shifting issue. Now, after it slips into forward or Reverse, it never slips out.
PS: If you go this route, remember to readjust the cable to compensate for the extended shifter. I had to do the adjustment an couple times after extending the shifter and then the problem was resolved.
If you look at the Hisun parts catalog (Alpha-Sports.com) it shows a cap for a carb engine and a different one one for an EFI engine. I'm certain they can assist you in getting the correct gas cap. It sounds like you have enough time invested now with the 400 that you should just fix it and enjoy it. Unless it had an excessive amount of hours on the hour meter or you find something major wrong, I think you did OK on the price. I bought a 2021 (in 2021) outfitter 550 and love it.
Don't worry too much about the steering wheel alignment, for now that is.
Do your toe in adjustment and while the outer tie rod end jam nuts are loose, you can take the six steering wheel cover off with a 4mm Allen wrench. Loosen the nut holding the steering wheel on and unthread it so it's flush with the end of the steering shaft. Use your knees or have someone lift up hard on the steering wheel while you hammer the nut/shaft to break it loose. I use two hammers. One ball peen hammer on the end of the nut /shaft and use the second one to hit the first one. A couple good whacks and the steering wheel will come loose. Then unthread the nut and pull off the steering wheel and realign the shaft splines and the steering wheel to make it straight again.
You may have to readjust your tie rods a bit to get true center on the steering wheel. I adjusted my toe IN to about 3/16" on each side.
Also, you'll have to release the two inner tie rod boot cover outer clamps so the tie rods can turn without winding up the rubber boots. Don't forget to reclamp the boots.
I said a lot of words, but it's really quite simple.
Good luck, Dan