[DiapDealer]

I believe process should be exactly the same if it's a stand-alone AZW3 or a stand-alone MOBI. Their header records (and EXTH) should be the same structure. The only time it would be different is if it's raw Kindlegen output ... which contains both MOBI and AZW3 formats (and a header and an EXTH for both).

If you are indeed talking about a dual-purpose file (MOBI/KF8), the thread you linked to wound down with the general consensus that editing the KF8's EXTH (in the second header of the hybrid file) didn't really solve much of anything (with regard to devices considering them Personal Docs and only supporting low-quality covers and such). It seems to be a bit of a wild goose chase.

In my opinion, the hybrid, dual-format output of Kindlegen shouldn't really be considered suitable for distribution. It WILL work on devices. But as you've found out, there are unsatisfactory side-effects.