I'd call Diono and see what they say, but also would probably bump the straps up to the next set if just slightly above the shoulders. I have some issue pullin our strap tight at the upward angle, but my button doesn't stay up...I also do not adjust after each in and out, I just leave it. And every couple days I have to tighten or loosen, which isn't too bad. It's just a little bit. I would not even consider turning a 20mo old, my 4.5yo twins (as well as both of my 2 year olds) are still RF. it may be a pain, but it won't last forever, and its worth knowing your kid is safest! I'd call Diono about the strap release button.
Sorry - I wasn't saying the button stays up. It doesn't. I was saying it lifts up and then instead of the strap tightening it won't tighten, bc you have to pull it down for it to tighten.