Just to update this thread in the hopes someone might have an idea on where I should focus:
1) Through a lot of trial and error, I'm able to get a BV to work, meaning, a lot of seating it, unseating it, restarting the machine, inserting and removing the cash box, etc.
2) It usually will work until I remove the cash box and put it back in, then all 3 lights on the MEI Cashflow go steady on and it's pretty much dead at that point (until I do #1 again).
I did another video of this behavior:
https://youtu.be/xN8IDcZmTt8Do you think it's possible that I just have a bad PCB I/O card for the MEI Cashflow 6602? I do have a complete new harness coming (BV, PCB and cashbox), but would love to isolate the issue since it's happening on two of my machines.