No, a Marlin 881 stock cannot fit in a model 81. The stock would fit an 881, not an 81.
No
No
No, the Remington Model 1100 stock has a hole (recepticle) through the inside of the stock in the pistol grip area to accept the action spring tube. The Remington Model 870 stock has no need for this hole so the 870 stock will not fit on the Model 1100 action.
This will not fit a model 12
No
I don't believe so.
Not without modifications
No
Yes
Any of the 124 series.
You will have to have to have a gunsmith check it out. Chances are it won't.