Well, you know the apocalypse is nighe when something positive happens in the world.
Today I received an email reply that they have fixed the technical issue at ebooks.com that caused my ebook preview to be structurally massacred. It actually looks pretty normal now —*if you don't read the content.
--- exhibit A:
19 May 2022, 17:37 GMT+8
Dear Roland,
I apologise for such a late reply.
Our developers found the issue and have now resolved it. The book is now displaying correctly in the Online Reader.
Sincerely,
[Tech supporter's name retracted out of my automated Canadian politeness reflex.]
Ebooks Support
www.eBooks.com
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I did create another CSS edited version of my ebook, but have only uploaded it to Amazon so far. But it definitely seems the issue was at their end and not with my CSS — although the code was definitely "heavy-handed" for ebook use.