I personally feel this is rather overthinking the problem. Can you point to any other Discourse instance where people are complaining about this, or this is even mentioned?
Because I can’t.
(Not opposed to a “jump to last unread” in the topic navigator, though.)
I don’t think this comes up often enough to be a huge complaint. That said, i do think this would be better default behavior… not a priority, and happy to let it ruminate.
Regarding the explicit “jump to first unread” in the topic navigator idea, I opened a separate topic here.