Maybe, but why? Why the "theoretical question"?
If you were allowed to enter, without having the immigrant visa endorsed then yes, the visa would remain valid until either used or it expires. You could also, enter USING the CR1 or IR1 visa, but not stay long. Your date of residency status would be established, and you would need to come back and start actually "residing" in the USA before within six months or so anyway. So, again, "why"?