I thought that the answer to #1 was no. I don't think it's consistent with the Amazon TOS to register to someone else's account for the purposes of "sharing" their books, and then deregister and register back to your own account.
I don't know if this is explicitly addressed in the TOS, but it certainly is contrary to the spirit. The multiple devices on one account feature is to allow multiple platforms (your Kindle, your phone, your PC - all running a Kindle app) and to allow those that live together to share books as you would physical books.
There is a sliminess factor to doing what is proposed above that makes my skin crawl.
Of course, your sliminess meter may well be calibrated differently than mine.