My understanding is that both versions of the unit (the original copy and the 2nd one after it dies) can both be revived in the infirmary, like any unit

That's correct, both can be revived with sapphires in the infirmary.  Any time you lose a feral swordsman (including in bgs) you get a feral phantom for free, and can refive the feral swordsman with saphs (at the usual 70% recovery ratio).