I am just curious what you find so punishing about either the Remington M870 or M1100? Every year my family has a Fourth of July shotgun shoot in which we go through a couple boxes of clays and an honest wheelbarrel of shells. Since my dad got his MEC he's been loading 1 1/8 oz of 7 1/2 to about 1300 fps(?) for this task and at any given time we probably have between 1000 to 1500 rounds of so of this sitting at my grandpa's.
I've owned a Remington M870 for about 4 years now and put an average of about 1500 rounds a year through it, mostly 2 3/4 inch small game loads, but also including 3 inch magnum goose, buckshot, and slug loads. I wouldn't ever suggest a steady diet of the latter as comfortable, but last year alone I put hundreds of rounds through my 870 during our family's shoot on my grandpa's place in the period of about an hour and a half. I was shooting so fast that the wood grip on my Wingmaster was getting warm and the barrel was so hot that I had to re-tighten my choke tube after every 3 to 5 rounds. My brother and I each had Wingmasters, my grandpa had his 1100, and my uncle brought his Benelli (which, for the record, gave a disappointing performance compared to the Remington, though that could be because it was the 18.5 inch barreled tactical model of the M1). My brother and I each put several tubes through the semis and they were noticebly more comfortable, but the majority of our shooting was done with our 870s. The four of us went through about 1500 rounds in under two hours, though my brother and I did most of the shooting as my dad, uncle, and grandpa all traded and cycled shotguns but my brother and I were pretty much always on the line. I never found the recoil to be a problem. Likewise, though I have much more limited experience with the 1100, what experience I do have tells me it has a problem with neither reliability nor recoil as both have been exceptional with my grandpa's.
Sorry to hear about your shoulder injury. I am not sure exactly how old my grandpa's 1100 is but he's had it for a while. It is a very easy shotgun to shoot. Also, my dad just picked up a 3 inch Model 1100 that he bought used but somehow hasn't managed to shoot in the four months or so sense he's bought it. It's the only one I've seen but apparently the 3 inch Model 1100 does exist. Good luck with your shoulder injury.