Incompatible Disk errors

There are a couple of threads on this which are closed, so I'm asking again in case anyone has had some success with a workaround.

I'm moving to a new Mac and so have decided to dedicate the old one as an Aperture workstation, running Mojave 10.14.6, but I also would like to run Monterey on it too.

To this end I started up from a High Sierra startup disk I made some time ago and formatted the drive as APFS with 2 volumes. I installed Mojave on 1 and via Recovery-Time Machine, set it back to 10.14.6 (backup from the day before it was updated to Catalina). This worked fine.

I had already installed Mojave on the the 2nd volume and I now upgraded this to Monterey, which automatically installed the last version.

As described in other threads, APFS changed in the interim between Mojave and Monterey and I now find myself with a broken disk. It works ok and I can startup on both volumes, but Disk Utility lists a lot of errors when run in Mojave (listed below). When run in Monterey, it's fine. So it seems that Monterey, during installation, changes things on the APFS disks and Mojave cannot run on a Monterey APFS disk.

Questions:

• Has anyone tried partitioning the disk as APFS instead of simply adding Volumes and putting each vs of MacOS on separate partitions?
• Has anyone tried using HFS+ to solve this problem? (Does it even work with Monterey?)
• If Monterey cannot be made to work, which vs of MacOS will remain compatible with Mojave flavoured APFS?

Many thanks for any help. Going through this is a veeeeerrrrryyyy slow process, which is why I'm hoping someone can help before I try the next step that occurs to me (currently making new TM backups :) ).

Disk Utility errors when run from Mojave install.

warning: drec_key: invalid hdr.obj_id (0x7)
warning: drec_val object (oid 0x7): invalid flags (0x205)
warning: drec_val object (oid 0x7): invalid type (5)
warning: directory valence check: directory (oid 0x7): orphan directory record
warning: drec_key: invalid hdr.obj_id (0x7)
warning: drec_val object (oid 0x7): invalid flags (0x205)
warning: drec_val object (oid 0x7): invalid type (5)
warning: directory valence check: directory (oid 0x7): orphan directory record
warning: drec_key: invalid hdr.obj_id (0x7)
warning: drec_val object (oid 0x7): invalid flags (0x205)
warning: drec_val object (oid 0x7): invalid type (5)
warning: directory valence check: directory (oid 0x7): orphan directory record
warning: inode_val: object (oid 0x1a5342): invalid internal_flags (0x2008000)
warning: invalid xf.xf_type (0xf)
warning: inode_val: object (oid 0x1a62f7): invalid internal_flags (0x2008000)
warning: invalid xf.xf_type (0xf)
warning: inode_val: object (oid 0x1aff0b): invalid internal_flags (0x2008000)
warning: invalid xf.xf_type (0xf)
Checking the extent ref tree.
Checking the fsroot tree.
warning: drec_key: invalid hdr.obj_id (0x7)
warning: drec_val object (oid 0x7): invalid flags (0x205)
warning: drec_val object (oid 0x7): invalid type (5)
warning: directory valence check: directory (oid 0x7): orphan directory record
warning: drec_key: invalid hdr.obj_id (0x7)
warning: drec_val object (oid 0x7): invalid flags (0x205)
warning: drec_val object (oid 0x7): invalid type (5)
warning: directory valence check: directory (oid 0x7): orphan directory record
warning: drec_key: invalid hdr.obj_id (0x7)
warning: drec_val object (oid 0x7): invalid flags (0x205)
warning: drec_val object (oid 0x7): invalid type (5)
warning: directory valence check: directory (oid 0x7): orphan directory record
warning: inode_val: object (oid 0x1a5342): invalid internal_flags (0x2008000)
warning: invalid xf.xf_type (0xf)
warning: inode_val: object (oid 0x1a62f7): invalid internal_flags (0x2008000)
warning: invalid xf.xf_type (0xf)
warning: inode_val: object (oid 0x1aff0b): invalid internal_flags (0x2008000)
warning: invalid xf.xf_type (0xf)
Checking volume.
Checking the APFS volume superblock.
The volume Preboot was formatted by newfs_apfs (748.77.8) and last modified by apfs_kext (945.275.7).