@ ard
You wrote: which ikar emunah does someone violate by calling you a kofer
Since you seem to be accusing SR of being disrespectful towards the Satmar Rebbe you probably meant to call his an apikoris not a kofer.
Second of all, the fact that a Rav of a particular community is held in high esteem by other Gedolim does not mean that all of Khal Yisroel must learn his seforim or follow his psakim and hadracha. And one certainly doesn’t have to follow the psakim and hadracha of his Chasidim. Asserting this obvious point, as SR has done, is not apikorsis.
If anything you ought to be more concerned about the blatant disrespect (both expressed and implied) coming from the other side.