Looking at the damaged bbl, you can see the pin groove and then the threads behind it are torn down /mangled enough to allow the bbl to unscrew. That damage from rolling over the surface of the pin being in place.
Likely the pin was chewed up as well,they are nothing special and are soft.
So:
I would suspect the 2" bbl was either forced into position the last few turns to pass by the still in place pin or what is left of it in the center section of it.
Or,,the threads on the 2" bbl were filed or lathe cut off from the breech end up to the pin cross cut so there would be no interference when threading the new bbl into place.
The bbl threads forward of the pin cross cut are sufficient to hold the bbl in place along with the torque of a good fit up of the bbl shoulder against the frame.
(Look very closely inside the frame at the bbl extending into the cclinder window. See if that small portion of the bbl has any indication of bluing missing from file work being done just ahead of the smooth section you see where the bbl threads start.)
That 4" bbl with the damaged threads, I wouldn't be too sure a die would clock onto whats left of the chewed up threads and recut them and be assured that it would follow perfectly onto the orig threads on the bbl.
There isn't much there to establish a start with the die,,maybe there is.
I would just neatly trim the damaged portion off as above and use the bbl that way.
The bbl pin doesn't really do anything anyway.
Also carefully check the frame for square, any bends or bend-back-in- shape repairs.
Removing the bbl by unscrewing it with the pin in place is a tough one.
S&W frames are touchy when things go right. The extra effort undoing a bbl with the pin in place must have taken some real effort.
Yes I think there needs to be a sit down with Mr. Gunsmith.