Yeah the rear seat belt latches are poorly designed. Even for an adult they aren't great but when you add in a booster seat it's effectively childproof. My daughter (and I) is happy with how the rigid 3" extender works. Easy for her to latch and unlatch and there haven't been any cases of the belt popping out.
Another thing I've done is to use some helicopter tape and have applied it along the door sil areas, specifically the weather seal that is inboard of where it contacts the door and then all along the plastic trim. My kids seem to use this as a step so I think this will go a long way towards protecting that area. I ran out of tape so didn't get to apply it to the body outboard of the door seal but have some so will do that also. I use the helicopter tape on my bikes and it is very durable and if applied well to a clean surface not very visible. I'm sure there are some auto type wrap products that would be as effective also. Here's the tape I used
https://www.amazon.com/ISC-Racers-Tape-HT1308-Helicopter-OG/dp/B000O65W9Y/ref=sr_1_1_sspa?ie=UTF8&qid=1543422458&sr=8-1-spons&keywords=racer+tape&th=1