Klugeryid and Reb Eliezer,
Bilaam was an individual after Matan Torah. So I think that example satisfies both of you. I don’t see why Ninveh being a large number of people should matter. If one non-Jew can’t do teshuvah, then 1000 non-Jews also couldn’t do teshuvah.
As for where the Seforno got his pshat, the Seforno is a border-line Rishon. It may be a mesora he got from his rabbeim. Or it might just be muchach from the posukim. Why would Hashem keep hinting to Bilaam to turn back and not go to Balak? Bilaam’s going wouldn’t hurt the Yidden because Hashem was going to make Bilaam say berochos not klalos anyway.
Another example of an individual is Yishmael on whom Rashi says (at the end of Chayei Sarah): יצחק וישמעאל. מִכָּאן שֶׁעָשָׂה יִשְׁמָעֵאל תְּשׁוּבָה וְהוֹלִיךְ אֶת יִצְחָק לְפָנָיו, וְהִיא שֵׂיבָה טוֹבָה שֶׁנֶּאֱמַר בְּאַבְרָהָם : This is also before Matan Torah but I don’t see why that should matter. If we know teshuvah existed for non-Jews before Matan Torah mehechi taisi that they lost it. Hamotzi m’chaveiro alav haraya.