It's definitely a playfield issue and not a cpu board issue if unplugging the idol cleared up the row short message. When you say you disconnected the idol, did you unplug J1 on the 3 switch opto pcb for the idol? If so, the LM339 chip (U1) on that board is likely bad and causing the problem. When an LM339 for optos fails, it confuses the swtich matrix and you can get all kinds of strange switch behavior from phantom swtich closures, double switch closures with optos involved to sometimes getting ground row shorts being reported.
I'm pretty sure you could confirm that it's the LM339 by unplugging J2, J3, and J4 on that board, but leave J1 plugged in. You're unplugging each individual opto on the idol when you do that (J2 is switch 91, which the only opto on the idol in row 1). If you unplug all those, but the row short stays, it's going to be the LM339. It's a cheap chip ($1.39 at Marco Specialties and $.59 for a socket) and fairly common to fail. I always put a socket on for the new chip when replacing for easy replacing in the future.
The switch row short is likely causing the 2 balls to be ejected during game play. Trough 6 is in Row 1, so if the pin thinks there's a short in Row 1, when the first ball is ejected, it will still see Trough 6 as closed when it should be open. Trough 5 will be closed too as it should be, so the game will spit out a 2nd ball thinking the trough is still full of balls (trough 1 - 6 all closed when it should be 1 - 5 closed and 6 open with a ball in the shooter lane). Once Trough 5 goes open because the 2nd ball is ejected and only 4 are now in the trough, the game will see Trough 6 closed, 5 open, and 4 - 1 all closed and know the trough isn't full.
The drop target issue I think can happen too if you turn the game on with the coin door open and the drop target can't go up and down because the coils have no power with the door open. If I recall the drop target pulses on power up. If the drop target can't move on power up, the game will think it's not working. Hitting it will make it go away. That switch is also in Row 1 though, so it may have also had to do with you having a ground short in row 1 message. I'd double check my memory on the drop target pulsing at power up, but my IJ is currently stripped down for a shop job.