The chamber length designation refers to the fired length of the shotshell. There is no question of that. The allowable chamber pressure for the 3-1/2" 12 gauge is significantly higher than the 2-3/4" and 3" loads so you cannot safely modify guns to take the longer shells unless you know what you are doing. Check SAAMI specs if you doubt it.
The 1200/1400 were Winchester's first attempt at reduced cost repeater shotguns, and they got such a bad reputation they helped lead to Winchester's demise, along with the disdain for the revised Model 70. If you have one and you like it, more power to you, and I hope it serves you well forever, but if the plastic feed throat or other inexpensive internal part breaks do not be surprised. The 1200 didn't last long, then came the 1300, then the Speed Pump, and now I think it has morphed into the Turkish made SXP (?). Every step did get a little better, and with FN now overseeing things maybe now they have something they can stick with.
Backboring, or overboring, refers to a bore which is larger than the nominal size (.729 for 12 gauge in the US). The choke is still at the front end of the barrel. Where are you getting all this MISinformation? Wikipedia has better info. BTW, my 1963 shotgun has over 115,000 rounds thru it, a lot of them maximum load handloaded waterfowl loads cooked up with Blue Dot and Ballistic Products components and it isn't a bit loose from "normal" wear.