Hi all, for what it's worth Arduino sent me a replacement H7. To troubleshoot they had me do the following things but it didn't seem to help:
- Updating Arduino IDE
- Installing latest core version of Arduino Mbed OS Portenta Boards from within Arduino IDE
- Trying to update Portenta H7 bootloader (board is not recognized so obviously I couldn't do this)
- Shorting the two pads on the vision shield pictured here in an attempt to force bootloader mode (shorting the pads didn't have any effect):
It took a while for them to believe me that the board was not being recognized on the COM ports 
but good of them to issue a replacement without too much fuss.