Just wanting to tell my experience now that Mojavehas reached with 10.14.6 its final issue.
I made two external clones of my High Sierra in HFS+ and upgraded one to Mojave APFS installing afterwards the 2 updates presently offered by Apple.
The second APFS clone was kept as it is following the suggestion long ago by
Fishrrman as what he called a “
Mule” Partition to be used only for updates.
I made an empty external partition in HFS+ and using CCC I cloned the updated Mojave to it making it bootable and obtaining a Mojave Recovery Partition.
I intended to boot from it and erase the inner HD in HFS+ but found that the Disk Utility of Mojave only allows erasing in APFS.
What followed was probably not the best idea.
I booted from an old external Yosemite. Maybe booting from an external High Sierra would have been better.
The Disk Utility of Yosemite did not show me the MacHD but a lot of strange, small partitions
I had no choice but partition the inner drive in 2 volumes, one formatted in HFS+ and the other in NTFS but lost by doing so the contents of my Bootcamp Windows.
At least I could finally clone the Mojave HFS+ external volume to the inner MacHD keeping it in HFS+.
At a certain moment my MP 6.1 crashed
and so did my nerfs...but it recovered thanks God!
Since Disk Warrior works with HFS+ (one of the reasons for me to want Mojave in HFS+) I used it to improve the MacHD volume correcting some faulty settings.
Finally using a Winclone Image I had kept I restored my Windows 10 to the NTFS partition of the inner drive.
As can be seen it was a complicated (maybe a bit risky) procedure but now I have Mojave and HFS+ in my MP 6.1.
The future Mojave updates from Apple will most certainly be only safety updates and I will try to use the Mojave APFS “Mule” drive for that.
To sum up, it was neither easy nor ideal but...possible.
Added on January 17. 2020
P.S. I must add that my Windows did not work as good as possible.
In the section of this forum for Windows and other OS I placed a thread explaining how I managed to install a fresh Windows without my not working Bootcamp Assistant, in case somebody has, as I had, a bad experience with that assistant.