And the answer is, yup, complex shaping is broken, as expected.
And for the record, here's Adobe RMSDK doing it mostly right on a Kobo H2O, and Kobo's ACCESS renderer doing it right (AFAICT) [provided an adequate font is side-loaded and selected, here, SIL Scheherazade].
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I'll check if I can mangle FC like on FW 5.x, but the hack is really, really not designed for that so it might be tricky.
EDITē: Nope, still no complex script shaping, even when very hackishly and quickly trying to replicate what works on FW 5.x. If anyone with more time on their hand wants to mess with FC, be my guest, but that's it for me

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On the plus side, I took the opportunity to check out Bookerly on small Pearl screens, and it looks very neat there ;p.