"Hebrew scholars I know tell me the word used for "earth" doesn't necessarily mean the whole planet, but could also mean just large geographical area."
Agreed. The more I hear of Hebrew scholars, the more I like them!
"If that were the case, then it could not have wiped out the whole human race. I'm more inclined to speculate that it occurred in Africa somewhere very early in human history at a time when the entire human population was concentrated in a small enough area so that it could be potentially wiped out by a flood."
But the entire human race wasn't wiped out by this localized event, else all of humanity would be inbred, mildly-brown-skinned descendants of Noah and his incestual children.
If this event happened 70,000 years ago, AND some of Noah's descendents interbred with sub-human homonids afterwards, then I don't think we would be experiencing the problems to which you are referring. Plenty of time for distinct races to evolve, and plenty of opportunity to introduce new genetic material into the human population.
Another possibility is that Noah and his sons represent clans or tribes, and the Ark perhaps symbolizes various means of escaping the delluge that God gave to the clans who would listen to him. Many individuals early Genesis can be interpreted as standing for groups of people, IMHO. Thus the delluge wiped all but the 2,000 or so people in those clans.