will a 11 or 12.5 inch barrel with a mlgs function reliably?or does it screw up the dwell timimg?

There are some 12.5s out there.

My shortest is 13.7

12.3" is the shortest you can go with a FSB w/ bayonet lug. 12.0" without a bayonet lug. Colt made 15.4" rifle gas barrels and 10.0" carbine gas barrels that worked alright so I don't see why a 12.0" mid length wouldn't. The problem is it won't work as well as something with more barrel in front of the gas port. When the rifle gets dirty or the weather gets cold you could experience short stroking.