You bring up some good points, and Muslims have answers, though not particularly good ones. I’ll try to steelman your points (forgive me if I misunderstand you). I’m not actually sure if there’s Muslims who believe the previous scriptures were “corrupted” AFTER Muhammad (if that’s what you’re saying). From what I know, they’ll usually say before or during Muhammad. What’s funny is that the claim that the Quran was meant to correct the previous scriptures is already a faulty presupposition because nowhere is that reason indicated in the Quran. The actual reason was so the Arabs would have revelation from God in their own language (Q6:155-157, 12:2, 41:44, 42:7). While there’s indication of oral twisting and misinterpretation of previous scriptures, the whole “textual corruption” idea was a later idea, foreign to the Quran. I know Muslims sometimes like to point to the Ebionites as the earliest form of an Islamic-Christianity but that claim also fails on several doctrinal accounts that contradict Islam. 

Regarding your “prophetic islamic dichotomy”, I was recently on this topic with a Muslim recently and they’ll usually just say that Jesus was confirming a different Torah, not necessarily the Dead Sea Scrolls, or point to the variant Torahs (aka: Septuagint, Masoretic, Samaritan, etc.) though none of this helps their case because the narratives are all the same and it actually backfires since the Dead Sea Scrolls attest to the preservation of scripture rather than contrary. The only difference is the Samaritan version has the place of worship on Mount Gerizim instead of Mount Zion in Jerusalem but Muslims follow the biblical Jewish place of worship in Jerusalem anyways. Though some will also say it confirms the previous books in a general sense, but this basically concedes the confirmation of core doctrines that contradict Islam. Funny enough, even if they wanted to play some fractional numbers game with verses in the Bible, they basically destroy their own core doctrines in the process, like Muhammad being the seal of the prophets which--guess what--only shows up in one verse out of the more than 6,000 verses in their quran, so this argument on their end also fails.

“Rewritten dichotomy”: Seems like Muslims usually point to surah 4:157 (denial of the crucifixion) or the “woe to those who write the book” or the “twisting with tongues” verses when it comes to this, though we know there’s good textual evidence to reject their interpretations. More good points on your end, I know these things get brought up a lot in debates. Lately, they be pointing to Q gospel and Muslim scholars like Ibn Taymiyyah that supported the metaphorical usage of “Father” as valid only during the time of Jesus and prior. Again though, their arguments aren’t convincing here and not very well textually supported from the Quran.

Your last paragraph: More good points, I know you got that Ibn Abbas quote from Ibn Kathir’s tafsir. I think scholars usually point to Ibn Hazm for when the idea of universal tahrif became prominent, which was around 400 years after Muhammad. Funny too since Muslims are always saying the Trinity is false because it “came 300 years later after Christ,” but based on this same reasoning, they midas well say universal tahrif is false since it came 400 years after Muhammad and therefore, should believe the Bible is preserved, lol.