Well fellas, it is now working…
Thanks @ andre.m for the tip on MFDeploy needing to match the current firmware. Based on your tip, I assumed MFDeploy 4.2 was also no good for a non 4.2 board. So I used an old MS MFDeploy I pulled from Microsoft to manually upload Config, firmware and firmware2 and they all worked (ignoring the signature error). Firmware2 did not die at the 60% mark anymore.
However, I’m not sure if this step was useful since the board still had the old 4.1 loader and a bad TinyCLR (according to FEZ Config, which indicated “Not available” or something).
Based on the tip from Dat, I focused on what Windows reported in the Device Manager when the Spider was in “loader” mode. One thread in the community indicated that other users (also w/ Windows 7 64-bits) were having USB device recognition problems, and needed to retry many times before a proper recognition.
So I went to the legacy updater FEZSpiderMainboardUpdater.exe. I switched the board to “loader” mode and reset it repeatedly until Windows recognized it this way in Device Manager:
COM12, GHI Boot Loader Interface
With this done, FEZSpiderMainboardUpdater.exe was able to push all files successfully.
Loader (TinyBooter) Version: 184.108.40.206
Firmware (TinyCLR) Version: 220.127.116.11
The Spider board now accepts an app compiled for 4.2.
Thanks Gus, Dat, andre.m and willgeorge – we’re now cookin!