Corvette: OMG DONE!

Good news, everyone!  The problem has been solved.
Remember how after the big disassembly shop job there was that one hella confusing issue?  I wrote about it a bit here, but I wrote about it more at pinside where I was hoping to get some help.

Short version:  the Route 66 kickout switch was fubared.  We replaced the microswitch + diode and now everything works.

Route 66 kickout assembly: switch and solenoid kicker

From the back, behind the rear wall on the playfield.  You'll note a missing screw, which probably didn't help.

Longer version:  when the route 66 kickout switch (switch #57) was hit it sometimes also phantom triggered the Trough Eject switch (# 37)
The Trough Eject switch is above the ball trough eject, and I guess detects if balls are jammed in there on top of each other.
When activated, the game tries to clear the trough by weakly pulsing the trough upkicker, and then strongly doing so.
When the Route 66 gate opened and the ball could then could sit in the kickout, a mode like CMIYC or Drag Strip Racing would begin, all the while the game is receiving continuous notification of the Trough Switch, and so while the modes are starting the game was clearing the balls out of the trough in to the shooter lane.
That is why the trough eject was firing even if there were no balls left in the trough.  It would keep firing until the Route 66 Kickout sent the ball out and around to the left inlane.

I am not expert on switch matrix issues, but testing all of the other switches in the rows and columns there was no other bleed through, just hitting the Route 66 kickout switch mistakingly triggered that other switch. 
So a new switch and diode was put in, problem solved.

Old switch and diode.

More rambles:  My first instincts with this was the optos.  This never happened before shopping the game, and happened immediately after the game got back together.  I then noticed I hadn't plugged the route 66 ramp opto connector in, and in the next game, everythiung was fine.
THIS WAS A TOTAL COINCIDENCE.  The switch matrix problem was intermittent, probably due to the switch not being attached properly behind the rear wall, and sometimes getting stuck.
Lesson learned:  Always check everything behind the back wall of a game as well.
The issue became crystal clear when, with the game on but not Started, touching that back switch causes the trough to kick.  So obviously then it was a utility function, and not indeed the gameplay code getting confused in any matter.
Good riddance, persnickety problem!

Trough opto board.  Notice the upper right LED is labeled JAM.
This is because the designers are HUGE Space Jam fans and that is for slamming a nasty dunk.

Oh, and the back gate of the LT5 wasn't raising.  Why not?  The spring wasn't connected.  Again, another solenoid sticking out the back.
Time to PLAY.