Techias hamaysim min hatorah refers to a concept that’s medeoraysoh, which is the drasha of az yashir. I don’t know what makes you think rashi is referring to anything other than that; the mishnah would include one who believes it was taught by neviim to be in that category too, as that is not included in the term “min hatorah.” Rashi is clearly saying that denying that particular drasha makes one a kofer.
The radak calling something a taimah is no different than when tosfos asks a kasha on a gemara. No idea where you want to go with that.
Rav saadya gaons perush on chumash was written in Arabic. There are also parts before and after that sniper that are missing from our text.
And rav saadia was not the only one who criticized the belief that the mabul did not affect eretz yisroel, and he was not the sharpest in his expression; the Ibn ezra says that people who think that are chaseirei daas, they are missing in daas. There is obviously no way the ibn ezra would refer to rebbe yochanan as a chaser daas, chas veshalom. One of the meforshim on ibn ezra, rav shmuel matot, writes that the ibn ezra(and we can say rav saadya, as he does not mention rebbe yochanan by name, and doesn’t even call him a chacham, rather just one of the nation) is referring to people who say that the mabul didn’t affect eretz yisroel at all, whereas rebbe yochanan agreed that there was some effect. Many medrashim also say like rebbe yochanan, so it’s not just “one from our nation.”
Further proof that rebbe yochanan agreed that the water touched eretz yisroel was from his encounter with a kusi, mentioned in bereshis rabbah, who said that har grizim was not flooded, whereas other mountains, including har habayis, were. Rebbe yochanan could have said that har habayis also wasn’t, if he held totally that eretz yisroel was not touched.
I haven’t heard of this piece from the abarbanel, but that’s defective at best. He goes to lengths to defend the ramban and say that he was not arguing with chazal. Period.
Re, the rambam; what other people could he have used as examples? Tzadok and baysus actually did begin with non halacha drashos, as it happens, because their issues started with denying olam haba(al tehiu k’avadim etc…)
Regarding rav shmuel bar chofni Gaon – not familiar. Maybe he means that you’re allowed to explain the pesukim in ways additionally to the kabalos and drashos of chazal; just speculation. Where does he say that?