I may be missing something - if I am please forgive me. *You said the
fan/light combo must be on a dedicated circuit, but then you said that
I could catch the GFI for the fan/light off the outlet run, implying
that now it's on the same circuit as the outlets.

If you have just a fan/light combo, it does not need its own circuit.
If you have a fan/light/heat combo, then you do. Just the heat part of
it will use at least 10-12 amps.
Whatever you choose, if its going to be inside the shower area it
needs to be GFI protected. If you have the fan/light just grab it off
the GFI outlet. If it has heat also, run a dedicated line and put it
on a GFI breaker.
If its outside the shower, it does not need GFI protection.

And just wanted to confirm - I shouldn't mess with trying to fish the
15A cable out and then fish a 20A wire down via the attic? *Did you
think that I should run across the crawl space and then up the HVAC
chase two floors?
The 15A cable might be stapled, unless you know for sure you can pull
it than go ahead. Otherwisw just run in the crawl space then up the
chase.