I think that the 10.14.5 supplemental update got installed on you Mac mini. Does your Mac mina boot from natively as I have a mid 2009 iMac 9,1 and a mid 2009 MacBook 5,2 and had to use Dosdude's APFS Rom patcher to enable mine to boot from APFS 2. when you hold the option button while booting does you ssd appear as a boot option if you only see one EFI and your usb Mojave patcher most likely there is an issues with your ssd or its formatted APFS and it is not recognized because its APFS if you can remove your ssd and install it on a working Mac format is HFS(extended Journaled and see if that works then try installing Mojave . 3 Have you used the APFS ROM patcher is is very tricky and could brick your Mac
No. ....I never used the ROM Patcher
and
Yes, Option booting with the latest patcher Only shows (1) EFI and the macOS Base system on the patcher. No SSD shows.
and
Since I have a time machine backup, as of three days ago with v10.14.5
WHY wouldn't reformatting the SSD as APFS Not be superior, to formatting it as HFS+ ?
And No,....I do not believe a late 2009 Mac mini, would natively boot off of an APFS formatted SSD.
Thank you.