Good point on turning down and bushing the firing pins.
I'd ask Tim's opinion on regulation and liner length. My approach would be to O-ring full length liners in on both ends and make muzzle bushings that thread on the liner and could be re-made eccentric if necessary and castellated on the end so they could be tuned with a spanner or choke wrench without rotating the liner itself. If you really wanted to be crafty, thread the muzzle of the liner back about an inch, make a 3/8" long nut that O-rings to the barrel for center and regulation (eccentric if necessary, or have a selection of them made), then have 1/2" long flanged muzzle nuts made to pull the liners in tension inside the barrels. I don't think there's any chance a short liner is going to make it more quiet, and none of the chamber insert makers have been indicted for making "suppressor parts" in the case of a 6" .22 LR or 9mm insert stuffed in, say, a 34" 12-gauge tube, so I wouldn't worry about that. Only legal issue is 18" is minimum barrel length even if you have it totally stubbed out or barrels completely replaced with metallic cartridge barrels...once a shotgun, always a shotgun unless you paper and engrave it as a short shotgun or AOW.
For extraction, I'd just put a fingernail relief in the face of the liner and either remove the 10-gauge extractors or clearance the rim of the liner for them so they don't interfere with each other.