Remove the single wire going to the 100 point relay coil. If the game now resets properly, you've verified the reset circuits are good. Next, troubleshoot why the wire you temporarily removed has power if all playfield switches are verified as open.
Use standard electrical troubleshooting techniques....

I've had the "end of stroke" switch on a 10's score drum unit bent in a way causing the 100 point relay to energize. Since the switch is part of the 100 point relay coil circuit, I assume you cut/desoldered it already. If not, try disconnecting it.
Ive also had the screw insulating tubes on switch stacks (on score reels) fail causing strange problems such as this.