I learnt that Abarbanel is the only commentary who disagrees with chazal in saying that David sinned with eishet ish, since chazal say "kol ha’omer David chatah, eino elah toeh". My teacher basically dismissed the abarbanel by saying that in his introduction, HE HIMSELF writes that he wrote his commentary while on the run, without access to chazal. Is this accurate? Can we really just discredit abarbanel in this case/ in general? I would feel really uncomfortable just dismissing a legitimate opinion, if abarbanel is legitimate. Thank you in advance!

Just as there is a disagreement among the commentators on this issue, there is a disagreement among chazal, as well. On that very daf which you quoted (Shabbat 56a, as well as Sanhedrin 107a) some say that David sinned with BatSheva, and others disagree. He isn’t alone on this issue among the commentators either; for example, see the Radak (11, 4) who cites both opinions. In general, teachers often try and simplify issues for educational purposes, but as you “learn how to learn”, and begin to see varied sources, you’ll reveal a more complex, but intellectually stimulating and harmonic picture of the deep and beautiful Living Torah, which has fascinated all generations. The Abarbanel is a totally accepted and legitimate part of this spectrum, and the Beit Yosef, R. Yosef Karo, even calls him “The Great Eagle” (Or.Ch. 168). His commentary is cited by the greatest of rabbis, including the most conservative halachic authorities like the Chatam Sofer, Maharsham, Minchat Yitzchak and B’Tzel HaChochma.