@ CS
“It is an oxymoron to say the Rebbe is a Torah giant but his position here is kefira, because that would make the Rebbe and chassidim who follow him the opposite of ovdei Hashem, and you can’t then say that you respect the Rebbe as a Gaon etc.”
This is just plain wrong. For example:
In matters of Dayos
There is a dayah in Chazal “Ain Moshiach L’Yisroel.” There are 2 ways this opinion is dealt with. 1) The Ran explains that it does not actually mean what it appears to be saying. 2) That it does mean what it appears to be saying and is the opinion of a Yachid which is rejected. Now, if someone today would like to understand this chazal kphuto and adapt this belief for themselves they would be kofrim in one of the yud gimmel ikkrim.
In matters of Halach
If someone today would follow the opinion that you can bmchalel Shabbos for Makshirai Milla such a person would be considered a mchallel Shabbos and we would not drink their wine.
In both of the above cases the proponent of the subsequently rejected opinion wa a Tzadik and a Gaon.