Rashi's Father at the Start of the Torah

On Rashi's opening comment on Bereishis 1:1, Rav Yitzchok said that the Torah need not have begun until the verse about this month being the beginning of the months, and the reason it begins with creation is to show that Hashem gave Eretz Yisroel to whomever He saw fit. In Divrei Dovid the Taz writes that in his youth he saw a very old manuscript commentary on Rashi that said this Rav Yitzchok is not found in the Midrash or the Gemara. The question itself is in the Midrash, but not in the name of Rav Yitzchok, who is none other than Rashi's father. Rashi, the Rav Shlomo son of Yitzchok, honored his father by placing him at the very start of his commentary. So there is no problem that Rashi seems to ask two questions, since the first was his father's. The Taz adds that the manuscript's claim that Rashi's father was no great scholar must be mistaken, because in his commentary on Avoda Zarah, page 75, Rashi quotes his father's explanation as the better one, saying that this is what he heard from his father and teacher and it seems correct to him.