My plan is to try a 10k resistor (guessing on the value here) inline with
the tip of the interconnect rca before the EQ.

Check your amp's manual. A lot of times there's a value for input
impedance. It's generally in the 10-30k range. Then just treat it as a
voltage divider.

PS Are you saying that this output isn't controlled by the volume knob?


Is this plan valid???

Another idea, better, would be to able to sum the mids & highs for the
back
speakers *without having them be summed to the front speakers. That way,
fronts and rear would be controlled by the head unit's volume control. I
don't know how to build this circuit, other than buying a little mixer.

It's a pain. Leave it alone.

Why not run just the low output to the rears? Where's the cutoff?