When flipping buildings over, is it possible for there to be 9 units per borough.
If so why would anyone stay/move there? Wouldn't they be danger zones? If so how would the units ever be killed. No one would want to go in to kill some of the units as it'd be too risky?
The first question you're missing is "Who would bother to stay there long enough to destroy all 9 buildings?". It would be a danger zone a good deal before it got to a full 9 units.
As for how to get it there/how to fix it, just find purchase cards that start with "S":
Seismic Ray: Spend 1E to use your [destroy] in any borough other than where you are.
Stink Attack: Whenever you move, you may disperse up to four units from your destination borough...
Subway: ...you can move as often as you like, whenever you like.
Super Speed: You can have a free move before rolling your dice.