@snarkophilus,
Welcome to the world of font wrangling on Kobos.

I seem to remember both of us were active on similar threads in the Sony days.
I'm a bit lost in the maze of your findings so far but here are a couple of old links which might have relevant info now or in the future.
- Getting monospace to work in kepubs
- Even if you're reading standard epubs rather than kepubs you may still be butting heads with the fact that the default epub override CSS inside the firmware contains:
Code:
body, p { font-family: -ua-default !important; }
which has the effect of ignoring any font-family styles set at the <p> tag level, in favour of your font-of-choice. For those who want it, there's a kobopatch ('Un-force font-family override p tags (std epubs)' to replace the body, p with a simple body.
- The general bold/italic problem with sideloaded fonts on Kobo, as already mentioned by @rtiangha. Posts #2 and #9 in this thread have detailed instructions on fixing the panose issue.