1. I don't see Plesk having addressed this anywhere in the thread until after my post. For that reason, I had no idea that this had been addressed. And I also have no idea why my recent Plesk servers still have the issue. Perhaps they are somehow out of date.
2. My work-around is very...
I could be mistaken, but I believe the following will "fix" a Courier-to-Dovecot "migration"/"conversion"/whatever, specifically with respect to IMAP "subscribed folders":
cd /var/qmail/mailnames/
for i in $(find . -maxdepth 3 -type d -name Maildir); do sed 's/^inbox\.//gI'...