After shooting thousands of rounds through my M1/M2's with RMR's mounted for 3-gun, shotgun-only competitions, and training (both giving and receiving), I can provide the following advise based on experience. Quick release of modern optics is overrated. Not once, in all this time of at least 12-15 years (including the early RMR's that had issues when mounted on handgun slides) I have never had a RMR go down on a shotgun necessitating its quick removal. I know that's only an example of maybe 5 out of all the mounted RMR's out there but it's still my experience.
Re: cluttered co-witness sight picture: If your sight picture is cluttered up when using a co-witnessed RDS, IMHO, you're doing it wrong. While none of my SG red dots are co-witnessed with irons, they are on some of my rifles. When actually shooting for a purpose, I don't even notice the irons in my sight picture. That's because, when shooting with a RDS, your sight should be focused on the target. The red dot is superimposed on the target because you're shooting with both eyes open (should be, anyway) and not trying to focus on aligning the irons. The irons just disappear and you're looking at a target with a red dot on it.
Again, that's me. (and how I train others)