[scruffs]

If I had any hair, I would be tearing it out.

So I open up a ePUB with Clibre. Add fonts, the manifest updates, etc., and paste the required css. Save and open in KP3 and everything is fine - the fonts are shown.

Drag the ePUB to Apple Books and the custom fonts are lost.

Getting close to the point where I just post the book on Amazon and forget about Apple. But I know one of you experts will have the answer for me. In exchange for virtual whiskey vouchers, of course.