What would be the cleanest method of converting my APFS macOS partition back to HFS+?

Once you formatted an HDD as an APFS volume it is not easy to get back to HFS+. I made the experience when I formatted an external 10 TB backup disk as APFS just to decide that maybe HFS+ would be better for my usage scenario. When I tried to reformat the HDD, the HFS+ version was gone from diskutil. I had to erase the complete volume structure and as soon as I did this, I was able to format as HFS+.

So I guess you will have to go through this too.

1. Full backup (time machine, carbon copy, ...)
2. Following my experience I would say to completely erase the volume structure of your internal HDD, through target disk mode or while booted into the recovery partition of an external USB drive.
3. Reinstall
4. Restore

Good luck!

With other words: I would say there is no way of avoiding a complete erase and reconfiguration of the whole volume structure + reinstall.

The “too many corpses” hinds at a corruption of your account/machine setting. If it is the user account you could try to create a new user:

1. Start with ⌘-S
2. Check the startup disk:/sbin/fsck -fy
3. Mount the startup disk using: /sbin/mount -uw /
4. Remove the setup complete flag using: rm /var/db/.AppleSetupDone
5. Install macOS and create a new user.
6. Copy your data to the new user.

If it is a machine setting related problem: erase, etc..