Waiting Hawk, Good for you for getting ready to go for it.

It sounds like at least some of your plan would be well outside of the National Electrical Code? The A/B situation using connection boxes the attic is probably a no go.Each branch circuit box is allowed only one power source.

What kind of inverter were you planning on using? Where are you going to tie it in? Sounds like you would be having it in your shed and be using it in a manner that attempts to isolate it from the grid?