You only need one cable, all 16 channels go down the same cable.

The more I hear about the issue, the more I am certain that it is a problem with the Yamaha. It should only be responding to one channel. Maybe three channels if it is set up to do upper, lower and pedals. You need to look at the Yamaha‘s MIDI section in the manual and figure out how to either turn off the lower and pedals parts, or if that can’t be done, You can set up in the EA7 which parts are transmitted.

But if it is literally responding to every single channel the EA7 is sending, is it making noise from the accompaniment parts as well, or just the keyboard parts?