Thanks for the extensive write-up—and demonstration that SAS HBAs are not expensive.
Two comments:
Flashing firmware is best done from the UEFI shell. Just download the package for UEFI, put the firmware and EFI tool on a FAT32-formatted USB thumbdrive and boot to to UEFI shell. No issue with root, no messing with a live ZFS pool, no need to disconnect drives.
As you found out the hard way, many server cards come in low profile (half-height) form. Some cards are sold with both low- and high-profile brackets; used cards may come with only one bracket.
If customs are not too much of a hassle, Chinese sellers on eBay must have the high-profile bracket you need.