The only way to move her house once she's officially placed the plot is to move her out, do the 16 villager cycle, and get her again.
In the future, you could/should plot reset for where her house goes so you'll have more control over her location.
Unfortunately not. If you have saved on a new character or loaded up your mayor the house plot is saved and now permanent. You will just have to work around it or move her out I'm afraid.
Any villagers that have recently moved out (the most recent 16 villagers to leave your town) will appear on Main Street, so that'll be why Diva's there