The Russians removed an older stock serial number (from an earlier rework) in a very crude fashion. Very common on RC that were reworked more than once.
I think the Russians kept them just in case they needed to import their brand of 'liberation' to other countries.
Possibly. It could also be that they were pulled out of storage and this was done in preparation for export, then they were restored. Who knows with the Soviets. They threw away nothing. I'm sure that there are some T26 tanks somewhere in an armory. I know they had T34s stored long after the war, much longer than they ever needed to keep them.