MySQL has been installed at last. Hit a few bugs along the way, but managed to figure out what was creating the errors that crashed the Berkeley DB to MySQL conversion.
The first problem was a typo in the EntryList plugin. Fixed that and tried again.
It then hit a warning caused by MT-Blacklist, solved that by temporarily removing MT-Blacklist and reinstalling it after the conversion. Even though I got the warning, the conversion happily ran until it stalled on the comments.
This time it turned out to be MTThreadedComments causing the error, it was installed but never used so the new fields never made it into the database. I never would have found it if I used the patch instead of manually adding the plugin additions to MT myself.
Some helpful advice from the MovableType Support Forum was to temporarily remove any offending plugins and turn off auto-discovery before running the conversion script.
The last problem the template files not having the proper permissions (no idea how they magically swapped permissions in the first place, sneaky files) chmodded them to 777 and we had a successful build.
Worth noting that when mt-db2sql.cgi fails, the tables that were partially generated need to be dropped from the database before it can be run again. Fortunately I have a very helpful ISP (thank you, Paul) who loaded the appropriate perl modules I needed before the conversion and killed the tables for me every time I hit a block.
So now we’re in the land of the far more stable and robust MySQL database, which will hopefully prove more Red Wolf-proof than Berkeley DB. But if it does get corrupted, at least there are tools to fix MySQL. Guess I’ll see how it goes.