Precisely my thought.
My take on that is that each GM selects what suits his saga the best, but each option come with a set of logical consequence:
1° PeMe "Mindless husk" effectively kill the spirit of the target, making it impossible to ever be restored: for all purpose the person his dead, leaving a body which is barely functionnal (it breathes, has a heartbeat - all function not link to the mind). Since the person is dead, Whisper through the Black Gate works and such. The body is really a shell and any attempt to put some spirit back into it should be treated similarly to the consequence linked to CrCo 75 Shadow of Life (I am not saying you need spell this magnitude to make the husk functional, only that there is chance of demonic possession and what not).
2° PeMe "Mindelss husk" is akin to a critical wound - I believe the parallel made by Darkwing is quite nice
. So it can be healed, eventually with time it can heal by natural means. Of course, the use of healing spell (CrMe equivalent to Chirurgeon's healing and all that) will granted bonus to recovery check or even fully restore the mental faculty (CrMe 45: Incantation of the Spirit made Whole).
For story purposes, it might very well be possible that the whole mages community believe in hypothesys 1. And mages believe that Whisper through the black gate fails to work because the spirit is supposedly so thoroughly destroyed that no spirit nor ghost is left to talk to. Whereas in fact, the truth is 2°(and obsviouly, Whisper Through the Black gate won't work on a living person).
And usually, youngsters are known for not believing in what their elders teach them so it could be a young mage discover such "mindless husk" and either by charity or for more nefarious experiment take him in his lab and starts to heal/experiment on him (with some minor CrMe20 - spell that a mage just a few years out of apprenticeship can manage). The result is a very slow recovery, possibly with the mindless husk believing to be somebody else because of botches spells or botches recovery rolls until - possibly years later - his true personnalities and memories emerged. That could lead to a very interesting story. How was he really treated by his caretaker (is he grateful, does he seek revenge) ? Did is temporary character was very different from his real character ? What if he got married (still suffering from amnesia) and then discover that he married an enemy's daughter ?...