KindleUnpack can indeed create an apnx file from the Page section (if present). But this just all seems a bit like a long and winding driveway to a very small house to me.
KindleUnpack the AZW3 (using the apnx file) to get an epub with a pagemap file that kindlegen can use to create a dual purpose mobi/kf8 with a Page section that KindleUnpack can create an apnx file from (and give you a standalone mobi).
Seems like it would be much easier to just use the original AZW3/apnx, to me. If one NEEDS an old style mobi, I would presume one has a device it can be read on (and can utilize the new 'round-the-horn apnx file), so why not just use that device to download the old-style mobi and its apnx file to begin with?