“They were mistaken, since they believed something that happens to be heresy”
So you are alleging that nebach an apikores is not an apikores. Regarding paskening beliefs, I will quote from what I posted in the Daas Torah thread: “see the Rambam peirush hamishnayos Sotah 3:5, Sanhedrin 10:3, Shavuos 1:4, Sefer Hamitzvos Lo Saaseh 133, Maamar Techias Hameisim, and R’ Shmuel Hanagid’s Mavo Hatalmud (the last line) for a start” This is also relevant to the other issue that I brought up which is that if there was a dissenting view at some point, then you can’t KNOW which view was correct.