I don't know the answers to all your questions, but these are the ones I do.
2) No. Syncing requires Whispernet, as does annotation/highlight backup. You can extract your annotations using the free
MBP reader perl script, which will export them to plain text. Only for Mobipocket-format files, but also usable on highlights from those non-Amazon-bought books which Amazon won't bother backing up for you.
3) Yes. People seem to get dinged for switching back and forth too often, or not having the pieces of their "story" match. It's probably best to begin as you mean to go on, and keep up the façade utterly once you've got it in place.
Always use a US-IP proxy, use a completely different email address to register your secondary account, use the "Transfer via Computer" download option when buying rather than receiving your books via Whispernet (or just keep Kindle for PC as your default device [they show up in the 1-click options in alphabetical order], sync for new books with the proxy on, and strip the DRM before you load up your books on the actual Kindle).
3) Yes. I have another account now. The original one is for my main non-ebook Amazon stuff, and there's a newer one which I wish I'd established sooner, which is just for the sheer volume of free promotional books they offer, which were starting to screw up my recommendations (lots of stuff in genres I don't typically read or care about, but hey, free is free!) and cluttering my "Manage Your Kindle" page.
Actually, all things considered, if you're on Windows I'd say set up Kindle for PC as the only device on the other account, do as per 3) above, and never bother switching between them.
Safer all around, perhaps.
Hope this helps.