Why didn’t Rav Shach feel a need to say eilu v’eilu regarding the Lubavitcher Rebbe? We obviously see if your sheeta holds another sheeta is assur then there is no chiyuv to respect it as divrei Elokim.
Of course, chazal often argued in Torah. I guess we must say that in those arguments one Rav could see the perspective of the other Rav as holding some validity, or at least felt that the other Rav was based in emes. If you hold somebody else’s opinion is based in sheker then you don’t need to respect it as divrei Elokim.
It’s not our call to judge whether an opinion that comes out of the mouth of someone respected as a Gadol is based in emes or sheker, but if another Gadol states a judgement on an opinion as being sheker then we have the right to stand behind him.