I think contemporary people (especially Chassidim) apply Sod Hashem Lireiav to Gedolim today.
That’s not muchrach, though. Applying it to Chaza’l is, though.
I think you need to prove that Sod Hashem Lireiav means they know everything.
I never claimed such a thing. It means they knew what Hashem chose to reveal to them, which may very well be only things which are halachah l’maaseh, but it would be mistaver to say it applies to all things recorded in the Mishnayos and Gemara.
I had a Rebbe in 8th grade who said it’s Apikorsus to say that Rishonim made a mistake.
I agree that the term is used too loosely. Technically, the word apikorsus actually means, I think, “hefkerus”, which it may be. I know some people who, if they don’t understand a Rishon in a sugya, will simply say he was wrong. That is indeed hefkerus. It’s not kefirah, though (which is how the term is used), it’s just stupid.