Ultimately should I create a dual boot install on the same ssd with two partitions (e.g. macOS High Sierra / macOS Mojave) and copy / paste the com.apple.telemetry.plugin file from High Sierra to Mojave?
I'm currently in a similar situation (dual boot Sierra / High Sierra on the same SSD) but both partitions are HFS +.
Is it possible to format the "Sierra" partition with APFS and install Mojave without deleting the existing "High Sierra" partition?
I managed to install Mojave on a system similar to yours. Here's what I did.
Latest Unibeast made under working High Sierra to make the USB stick. I had no success booting from this, so using Clover to mount the stick's EFI partition folder, delete the contents, and install the latest Clover build (I used Clover_v2.4k_r4695), choosing the default settings for legacy install (not UEFI booting), and ApfsDriverLoader-64.efi in drivers64. The rest I left as is.
Installed this to a separate old empty hard drive (not ssd), formatted in disk utility under APFS (wasn't sure this would work, but did). Once install was done (it'll reboot a few times during the process, make sure to leave the USB plugged in and choose the Mojave drive once it does to continue install), I couldn't yet boot into Mojave because our old cpus don't have SSE4.2. Workaround for that is boot back into your working High Sierra build, open the Mojave drive, go to /System/Library/UserEventPlugins/ and paste in "com.apple.telemetry.plugin" from High Sierra version 10.13.5 (google it, you'll find it, other HS version might work, but this one worked for me). Reboot from USB, choose your new Mojave drive, and it should now work.
Once booted, use the same version of Clover r4695 make your install bootable, choosing the same options as for the USB. Additional things I needed was to copy over my "Other" kexts from my High Sierra EFI, and modify the SMBIOS to version 14,2 (open config file in Clover Configurator and change).
Other things I needed were to install Lilu.kext and NoVPAJpeg.kext to get Quick Look working correctly. Place a copy of both kexts on your desktop along with KextBeast.pkg, run KextBeast and install them to /Library/Extensions
From there, once it was working as it should, I migrated my High Sierra contents to this new drive, then SuperDuper copied this new drive back to the SSD, installed Clover again (with all above options) to make it bootable, as that's it.
Hope that helps.