With power off see if you can manually get the 100 reel to advance by pushing the plunger in, compare to the good reels. My guess is the reel won't advance because the plunger won't move, and the plunger won't move because the coil overheated, melting it in place. Probably due to a 100pt playfield switch or 100pt relay stuck.

I was going to say the same thing, 100pt looks energized to me. See if it bounces back when you power it off. The second problem you have is the game should start in game over when turned on so the playfield switches and scoring shouldn't even be energized.

I was just thinking the other thing that could hold in the 100 relay is the 500 point scoring circuit. I don't have a schematic but I believe there should be a 500pt relay that works in conjunction with the score motor. That relay would hold itself in and turn the score motor to activate the 100pt relay 5 times. That switch on the score motor switch might be bent permanently closed.