Joseph,
Saying that the dayanim are posul is different from saying that the geirus would be no good if the convert himself believed. The only theological test for a Ger that we find in Shas is kabolos ol mitzvos. Lo motzinu that there is a requirement to affirm the ikkarim of Yehadus, let alone something like this which is not even an ikar.
Even l’gabey the dayanim, did Rabbi Eisenstein say this to pasul geirus done by those dayanim retroactively or only m’kan u’lhaba?
I am still skeptical of this psak (without an actual teshuvah with sources) but this is significantly more limited.