I can't think anyway off hand to do conditional CSS.
The CSS on titlepage being affected by the presence or absence of a cover--a separate HTML page--sounds like a reader bug to me.
Unless there's some processing step going on that combines the cover and title page, then I could see the cover_content embedded styles affecting titlepage.
I assume we've already told you about the
cover_content and titlepage_start settings.
If that's the case, embedding your titlepage CSS using titlepage_start might help, or changing the embedded CSS in cover_content.
If either of those help, I'd be willing to experiment with adding the changes to defaults.ini.