@NiJuLe (and thanks again)
FYI
After looking at your sources I got the font mapping working (with this in the alternate.cfg):
Code:
<match target="pattern">
<test name="family"><string>serif</string></test>
<edit name="family" mode="prepend" binding="strong"><string>MyFavSerifFont</string></edit>
</match>
<match target="pattern">
<test name="family"><string>sans-serif</string></test>
<edit name="family" mode="prepend" binding="strong"><string>MyFavSans-SerifFont</string></edit>
</match>
The "override" of sans-serif affects the UI of course - but it looks good (better than with Futura) and I haven't encountered adverse effects yet.
The "override" of the fallback font I found in your sources I'm reluctant to use - easier to edit the offending ebook.
But I failed to persuade the Kindle to accept a binding/mapping of the font-families "cursive" / "fantasy" to any font - so another one for Calibre's EditBook feature.