FWIW, it looks like the key to this is a pickle. Specifically it's iterator.pickle.
https://www.mobileread.com/forums/sh...d.php?t=265864
While the current page and the bookmarks are stored in the epub, they're also stored in iterator.pickle in ...users/{username}/appdata/roaming/calibre/. They are taken from the local location preferentially.
I copied the data from that folder to a folder in the cloud where both PC's could see it and then created an environmental variable CALIBRE_CONFIG_DIRECTORY on both machines pointing there. After reboots (and multiple episodes of correcting poor spelling) it seems to be working.
Fingers crossed.
Thanks again to those who helped me here.