Chuck,
I cant answer all of your questions but having just returned an M1 Super 90 to a gun shop that I bought it from due to cycling problems, I can say to keep a wary eye out...mine would NOT cycle more than one 2 3/4 shell unless I had it planted so hard into my shoulder that I could never track a bird comfortably, thereby making it useless for hunting. I promptly paid the difference on a brand new M2 Field and dont regret it for a second....Night and Day difference. That being said, yes, it does have the inertia system (and it a semi-auto, not auto, but I think thats what you meant)...I dont know about the gel pads the stock because the newer "comfort-stock" set up is a whole "system" of parts if you will and I cant say the gel pad alone could be popped onto the back of the M1 stock....the other tough part about the M1 is it has the ability to be "ghost loaded" so even with a 3 shell plug in, you could drop on more on top of the elevator and essentially have a 4th shell in there...The game wardens are fully aware of this and will check you on it so you need a longer plug and then, sadly, hafta put one in the chamber, one in the tube and the 2nd shell in limbo on top of the elevator without engaging it....its a royal PIA...The M2 has no problems like that. So basically, its a fine shotgun but they seem to have cycling issues and would probably be a better tactical gun. For the right price, it may still be worth it tho.
Robb