Thank you Mara - the CSSU-testing thread pointed me in the right direction. The package manager gave me a /etc/default/mount-opts.dpkg-dist file, which apparently should overwrite mount-opts.

"mv -i mount-opts.dpkg-dist mount-opts" fixed everything and has returned behaviour to normal.

I can also confirm that the delay is indeed due to fsck.vfat running - thanks for that tip also!

Edit:
Added bonus - this has fixed Backup too!

The only remaining mystery is what "dpkg -s mp-fremantle-community-pr | grep Version" should report. But that's not terribly important at this point, because my tangible issues are fixed...