I can't answer your other questions, but deregistering your kindle is certainly a valid option. You can side load without a problem.
So long as your books do not have a DRM, they can go right onto the kindle. I lend my 6 inch out occasionally and deregistering eliminates access to my Amazon account. The person can still read the books that are already on the kindle. When I get the kindle back, I can simply reregister it.