nice—i've never thought of using multiple adjacent sibling selectors like that.
i realize this doesn't really answer your question, but i've used simple h2 + p type adjacent sibling selectors quite a bit, and can't think of an epub reader that doesn't properly display them. it is valid CSS 2/2.1 (for what that's worth).
here's a test book in case the nice people on mobileread want to help you crowdsource some tests:
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