Thanks Jake. I did try your suggestion, but the same behavior happens despite which port is used.
The system is using BIOS A12, which should be compatible according to the guide.
The BIOS settings are in place, as outlined.
I know my USB drive is good, because I used it to also install on an E7440 using this separate guide: https://osxlatitude.com/index.php?/topic/6461-guide-e7440-with-clover/
My CLOVER partition has a clean copy of the Clover EFI folder for use with the E5530, as well.