I'll go to #3 for discussion purposes.
You used to be able to have a gun in your home without a LTC or FID, the law was changed after an incident in Roxbury of all places.
So if an unlicensed person managed to get hold of a weapon that under the Mass safe storage law there will be some repercussions for the gun owner, you can take that to the bank, unless it was a case of the gun owner was incapacitated by some means while defending the home, and someone in the home picked up the gun and fired as a matter of self defense.
It would be an interesting case as Bruen clarified that a law abiding citizen has the right to use a gun for self protection inside and outside of the home.
It would also be a case where the D.A. would kick the can to a Grand Jury and you hope and pray the Grand Jury returns a no bill.
But the choice of weapon has very little to do with the legal jeopardy, it is all laid out in the 23 pages in this link if you have the time to read it.