Yekke2,
That the arayos before the Torah were only maternal is because the arayos of the Sheva Mitzvos (which certainly applied before Mattan Torah) are only maternal. It doesn’t answer the kasha of the Shevatim keeping the Torah before Matan Torah.
As an aside what is the mekor in Chazal that the shevatim kept the Torah. The mekor that I know of says that Avraham Avinu kept the Torah, it doesn’t say anything about the Shevatim.
I once heard a pshat that before Matan Torah Avraham Avinu and his descendants kept the Mitzvo Essai but not the Mitzvos Lo Sassei. Before Matan Torah the Ivrim had the status of ayno metzuveh v’oseh and we don’t find the concept of an ayno metzuveh v’oseh for a lo sassei.