I'm posting in this topic again because just today I've had to test some books on Amazon and KDP once more stripped my fonts. But when I used the @import rule, it worked, again.
Now, I really don't know if I lucked out or what, but for me the @import solution really
does work all the time.
For testing purposes I've made a small validated epub containing a cover and 5 sentences each written in its own font. I've made 2 versions: one with a normally-linked stylesheet, another with the @import stylesheet. I uploaded both to KDP. KDP stripped fonts from the former, left the fonts in the latter. See screenshots below:
Normal stylesheet - KDP Online Preview:
@import stylesheet - KDP Online Preview:
The source epubs were made in Sigil 0.9.3. If someone wants to check and de-bug them and the resulting mobis, download them in a zip file here:
https://www.dropbox.com/s/j9rbrr31cp...0test.zip?dl=0