1) Each year you move natural increase plus a few % of the base. Much like amortizing a 25 year loan.
3) Yes, if you have all black counties, but all black states make it a lot more likely to be permanent. You need all black areas so you have jobs for the college graduates and high school graduates.
Now all this being said, the odds of it happening are not good. Funding is the big issue. I just can't see heavy taxes in New York and Boston to pay for the new black used infrastructure.
I think the big issue is freedom of movement and residence. These are now free US citizens, and the government doesn't have the authority to deport them en masse, or require citizens of any race to live in or be excluded from particular states. The vast majority of blacks positively did not want to move to Africa or the western frontier.
Setting up black states reminds me of the black 'homelands' that South Africa set up during aparthied. It enshrines segregation into the fabric of the nation.