NevilleChamberlain, name one genuine shita that holds bor al gabei bor is possul. There aren’t any. Any rabbi who “paskens” not to use such a mikveh is either an am hooretz or operating on sin’as chinom.
The one shita that opponents claim passels such a mikveh is the Divrei Chaim, which is ironic since they ignore him on every other topic, while Chabad regards him as a serious posek; but their characterization of his position is an outright lie. Bor al gabei bor is kosher and mehudar according to his shita just as it is according to every other shita.