“we know that thee Avos maintained the Torah, even though they lived before it was given.”
We actually don’t “know” that; there are many opinions as to what they kept, some of which were offered by HaLeivi above. The gemara in Sanhedrin uses the actions of the Avot to derive laws for Noachides, not Jews. For example, a Noachide may marry a paternal half-sister and Avraham Avinu’s marriage to Sarah Imeinu is used to prove that.