Your APFS to HFS+ converter appears not to work under OSX 10.14 Mojave. At least it did not on my MacMini. I got a "Failed to Convert APFS Volume" message after about 20 seconds. Same error after a second try. After that booting in Mojave was impossible. The Mac could no longer find the system.
Yes, I had done a dual backup, both in Time Machine and with Carbon Copy Cloner. So I was able to restore everything. But I still would love to do the APFS to HFS+ conversion one day soon. So please advise.