I would do anything I could to keep a 20 month old RFing, including buying a new seat. FFing at that age would not be an option for me.
If the metal piece that locks the harness in a tight position is not flipiping back down on its own, you may want to call Diono. I had a radian that did that and they sent me a new one; it's supposed to flip down on its own.
I moved DS's straps up to the first non-slanted slot when the straps were very slightly above his shoulders. Probably about 1/4 of an inch higher than in the picture pp shared. If they are slightly above, and not near the top of the shell, I feel comfortable with that as a parental decision (I am a tech, but that's a parental decision, not something I'd advocate as a tech). I think the main risk with shoulders below straps RFing is ramping up the seat; someone else please correct me if I'm wrong there.
Is the problem with the harness adjuster that it's buried against the cat seat back? Are you using an angle adjuster? You might try installing the radian somewhat less upright (here I'm assuming if the harness adjust is buried that the seat is very upright) to get the pull strap at a better angle. If you change the angle of the seat, it will also change where your DS's shoulders fall, so it might even help with that issue. If you can post pics, you might get more suggestions.