The extra 3-wire cable should remain disconnected. I believe it's for a fan sensor, but the RockyLake board doesn't offer pins for it.
You are correct that the BMC ethernet is the port which is by itself. The other four ethernet ports are currently unused. You are correct that the IP of the BMC is fixed and the preferred method is to have another host or a second network interface on the MCPC setup with an IP on the same network as the BMC.
I'll find someone who can explain exactly what the plans are for the SATA and additional ethernet ports.
SATA and GbE functionality is planned for 2H 2010. These will require an update to the FPGA bitstream. We don't plan a universal upgrade at this time, because it isn't clear how many researchers will require this functionality, but we will make details available as these functions become available.
Would you find these features useful? We can create a list for folks who need these for their research.