Mojave Bootable Install USB Problem on iMac Sonoma

On our 2019 Imac27", Sonoma crashed after the 14.3 update and we had to reinstall all, only to find that the Fusion Drive had split. After fixing the Fusion Drive, we then re-installed Sonoma 14.3 again from ™. So far, so good. To clarify, we had purchased 2019 Intel iMac 27" a couple of years ago to run under Mojave due to the fact that we have a version of design software that can only run on Mojave and no higher. So when we updated to Sonoma the scheme was to run the design software with a Dual Boot to Mojave 10.14.6. And we had been doing prior to the Sonoma 14.3 update.

We currently are trying to re-install Mojave with problems of the Bootable Install USB loading on the startup disk selection screen, and when run we get a black screen with a Zero and computer shuts down.

This time we created a seperate APFS volume (macOS MOJAVE) for the Mojave system. The Bootable Installer USB has been checked and indictes as bootable. Otherwise, Sonoma is running fine at this point.

Our problem is that we can not remember whether the earlier dual boot situation was to a Volume or a Partition and we would appreciate someone clarify which is right. We have seen recommendations both ways. Have looked into boot security and the 2019 iMac does not indicate any method of change to access external disks that we have found.

Finally, how do we get Dual Bootable Installer to run to install Mojave.

All help appreciated