Like I posted previously ask CS. No one seems to know. Or think what they know is correct.................
Well, having had a Kindle of one sort or another for 3½ years, and having been a member here for nearly as long, I am pretty sure that what I posted above is correct.

Though I absolutely advocate contacting Kindle Customer Service (see
this thread for phone numbers) if anything is not clear or not working as expected.
To summarize:
A kindle can only be registered to one account at a time.
As a rule, you can't 'combine' accounts previously opened to share books, though there has been at least one report of someone who was allowed to return books for a refund so that she could re-buy them on the second account. In this case it was only a handful of books and the Kindle was only a month or so old.
Individuals CAN share an account -- I share my account with my brother and we share the books either of us buys. Other members here report multiple family members sharing an account -- each, in some cases, with multiple devices.

If you de-register a Fire from an account, all content will be removed -- you can go through JetJammer's rather laborious process if you wish to retain it.
If two people are going to share one device, they have to pick the account it will be registered to. Now, the Fire is brand new, but in the past, when one member of a reading couple got a Kindle with the intention to 'share' it, it wasn't very long before they found the best thing was to just get a second Kindle.

Aussie, please let us know if this is helpful or of you have any other questions! And Welcome!