Hello John.
I have tried your configuration, but SymbOS3.1 starts correctly for me with only the SD-cartridge connected, and the EnterMice works also.
Maybe your SymbOS boot driver is now the USB SF3?
The EnterMice connector could be misaligned or with dirt, like all EDGE connected devices. Try to unplug and plug it again.
Also, after some seconds of booting, if no EnterMice is found, it is disabled, the normal behaviour.
Thanks for testing. Now is the time to find errors, not the next year.....
What doesn't work for me is the EP with only the SF3 connected, the pen-drive is not set as massive storage so, once booted, SymbOS is not usable. I am waiting for Prodatron to fix it.