1. Yep, that's what I meant by "portable"
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2. I've never used it (I'm not a heavy annotation user, period), so I'll let someone more familiar with it field that one
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I would assume that's what @ilovejedd alluded to earlier, but I have no idea how it works in practice
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3. b & c, IIRC. (Again, never really used Kindles with WiFi on, so, err, take that with a grain of salt).
As for a., depends on what you mean, exactly. If it's not archived in your Library, a *device* sync will pull a device-specific version of the book that ought to be compatible with existing hl/notes on other devices. I *assume* it's true even if one device gets a KFX and the other an old M7 (i.e., worst-case scenario, as far as format generations are concerned), but I may be wrong on that point.
4. May apply to sideloaded content if it's flagged as EBOK with the proper ASIN (which Calibre can do). Again, take that with a grain of salt, I haven't followed those shenanigans in a good long while, and I was never the target audience to begin with.
5. Can't downgrade current Kindles. No diags and no serial shell on a PW3 means you *probably* can't do it over serial either.
6. None, RP & CRP can't work on those devices.
7. Final 5.9.x (or the exact 5.10.x before the KFX DRM update, I'm not familiar with the exact version, I think @ilovejedd knows though
).