I would take the sound source from the output of the keyboard -- Stereo right and left, one to each speaker.

On the Kn7000 you can turn off the internal speakers – I am not sure how your keyboard handles this. I remember putting a dummy ¼ inch plug into the headphone output – I have heard but positives and negatives about doing it that way.
(a dummy jack is a ¼ inch jack that is not wired.)

No answers for your other questions.

John C.