What would our gravity on earth be if the moon had not been ejected and instead retained as mass? (According to this theory.) Moreover, what equation could I derive that from?
The change in surface gravity wouldn't be meaningful. Capillary action would still permit tall trees and chemical rockets would still reach escape velocity. Toss Mars or Venus into the Pacific, on the other hand, and a more interesting game develops.
[Edit: Note that the moon is less dense than the earth. By my calculations, adding the moon's material to the earth (and keeping it at the moon's density) would increase the earth's radius by 43 km, and (if you accept the parent's 1.2% mass increase figure), the surface acceleration would be 0.998 times the current value.]