So are we refuting the concept of Revelation here? or the concept of Covenant? (which did occur throughout Bereshit/Genesis prior to Moses).What I find strange is before the Torah there was no law. There is your dispensationalism. Tribes like the midians etc, God winking at pagans ignorance. The Torah was not some established law. Apparently that thinking doesn't extend to Christ. I see alot of illogicality in that.
Covenant is not dispensationalism, dispensationalism is a change in mode of salvation and the segregation of revelation into periods.
But to correct you there, as early as Noah we see him practicing similar things to the Israelites of Moses' day, with similar legal concerns, but the details of this are not given in Bereshit/Genesis itself the way that it is in Leviticus and Deuteronomy. In terms of the Torah's internal theology, it doesn't appear as in development (unlike seen via the later Prophets and the New Testament both to each other and to the Torah) but tends to be more in instruction in the way that the writer of the Torah intends to convey legal concerns through the stories, as well as their historical realities.
As for if you are going to use dispensationalist logic, I can't see why you can't also use that for Islam, hypothetically.