The machine does have a 64-bit EFI, see the specs here
, and it's backed by the fact that the machine does boot to Mint after the delay, and it's booted in EFI mode (since the OP can use efibootmgr).
In my humble opinion, I would just persist and solve the boot delay.
The content of the EFI partition is perfectly fine, so all files of a correct install are there.
I think I understand what happens, and the solution is to replace the CMOS battery.
The first piece of the puzzle is in the content of the EFI partition, there is this:
Code: Select all
EFI/BOOT/BOOTx64.efi is the location of the fallback bootloader. So if the firmware does not know what to do, it loads that.
In the same directory you have fbx64.efi, which is a EFI program whose purpose is explained here : https://www.rodsbooks.com/efi-bootloaders/fallback.html
The second piece of the puzzle is this message
Code: Select all
System BootOrder not found. Initializing defaults.
At first I thought this message was coming from the Apple firmware. This would be strange because Macs are normally pretty silent when having problems, they tend to communicate more by beeps, lights or screen signs which are rather cryptic. After an internet search, I'm now 90% sure this message instead comes from fbx64.efi.
So what's happening (I think) :
* CMOS battery is weak or dead, so the boot order gets lost at every boot.
* the firmware does not have a boot order, so it boots the fallback EFI/BOOT/BOOTx64.efi (which is really shim), which chainloads fbx64.efi
* fbx64.efi prints out the forementioned message and proceeds to recreate the boot order
* it finds BOOTX64.CSV in the EFI/ubuntu/ directory so it creates a boot entry for shimx64.efi, and since it's the only thing it finds, it then chainloads it
* EFI/ubuntu/shimx64.efi chainloads to EFI/ubuntu/grubx64.efi , i.e. grub which presents you with the grub menu from which you can load Mint. (grub does not really know how to launch macOS, that's another problem we will need to fix later)
* all of this takes some time to do, so that's why you get the ~10 second delay.
In fact I would consider you lucky. First lucky that fbx64.efi is there and makes its job correctly, and second lucky that it takes only ~10 seconds. (I have the experience of a Mac where the NVRAM was corrupted and was recreated by the firmware itself, and it took several minutes)
So in short, my opinion is that you need to replace the CMOS battery.
After you have replaced it, boot the machine. The first boot will go as the previous ones, with the 10s delay, the message etc. But then the boot order will be stored and will not get lost. So the next boot afterwards should get you more directly to the grub menu without delay.