Summa and PBA,
I went through Rav Sherman’s opinion quickly in between mincha and maariv. I will re-read it later but it seems to me that he is not disagreeing with what I wrote above.
Rav Sherman writes quite clearly that if a Ger is properly mekabel mitzvos before a competetent Bais Din he is Jewish and later conduct against Torah and mitzvos in no way invalidates his geirus, even if he says he never really meant the kabala.
Rav Sherman is arguing, however, that there was no kabalas mitzvos before a competent Bais Din, because the Bais Din that oversaw the geirus didn’t do its job trying to determine whether there was proper kabalos mitzvos. It was kabalos mitzvos before an improper, incompetent Bais Din. Rav Sherman is discounting the geirus on the same sort of grounds we discount Conservative geirus.