I'm really grateful for your continuing support!
but I guess we don't get each other quite completely
I have a working Mojave installation on my PC (in fact, I'm writing from/on it now) which boots fine with Clover 5122.
I wanted to try booting via OC, because updating Clover often has meant copious work till the boot works again, so I installed OC on the EFI partition and (for good measure) the Mojave recovery ("online installer") on the second partition, I do not really need this.
I try booting the already installed Mojave via OC instead of Clover. NVRAM is being reset before each try, kexts have been removed (and put back later) from /L/E and /S/L/E and the Clover NVRAM/SMC startup scripts are in place for the time being.
The boot of Mojave fails (reboots) so early that I doubt that the "spare" kexts from Clover are being loaded at all (actually, without them the same rebooting problem existed).
I don't want or need to install anything (except for OC)
You need to make sure your config.plist is set to show the Picker List, with a reasonable Timeout of at least 5 seconds, so you can select the correct boot option on the list. This will also give you time to select the ClearNvram option, which you should do before you boot the new USB installer.
The options for OC are "show picker" and "timeout 0 = disabled, wait forever". Nonetheless, without interaction the picker is not shown and it directly boots the abovementioned spare recovery system from USB. Only frantically hitting any keys after selecting "UEFI USB partition 1" in the boot menu gets me to the picker.
I made sure to be working on the "right" configuration file by changing picker colors (which is applied), but the "stay on picker" option apparently is ignored.
If you provide a copy of your current/revised OpenCore EFI folder, I can run my eye over the settings/contents to see if anything needs attention.
I use the EFI folder you already checked and modified, but to be extra sure, I'll upload it again with all current changes. I kept your changes regarding Coffee Lake configuration, but because the installed Mojave is working fine with iMessage et al, for the time being I kept the iMac18,3 system definition with respective serial/MLB/UUID...
Thanks again!