Probably a connector or header problem. Look for cracked through hole soldered headers. Do the relays click when they are supposed to? This will help determine if the problem is upstream or downstream from the relays - or determine if it is the relay trigger circuit or the GO string the relay is supposed to be switching. Use the coil test in diagnostic mode.

If you don't find the problem, let us kniw , this can be narrowed down with a DMM/voltmeter

If the GI relays are not clicking then you need to determine which transistor is the switch that grounds the trigger that completes the ground for the relay trigger signal. Ground that transistors mounting tab. If the GI works when you ground the tab, then the problem is the transistor or upstream from it. If it does not, the problem is downstream from the transistor.

Look at the solenoid chart in the front of the manual. It tells you which transistor controls the GI relays. In Cyclone, Q9 controls playfield GI and Q16 backglass GI.

To test, use an alligator clip test lead, one end clipped to a suitable ground such as a board mounting screw, and touch the other end to the transistor tab, briefly. As long you don't hold it on there long, you won't damage anything. Just a quick touch to test.

So when you grounded the tab, the GI lamps turned on, correct?
This test does not test the transistors themselmselves. it is to test the wiring and GI relays downstream.

Yes, the diagnostic LED should be continually flashing.

First, I would recommend not adding anything and focusing on one problem at a time. Let's table the ferris wheel for the moment.

The solenoid table in the manual show the pf GI is solenoid #10 and wiring is 8P3-10 via 5J2-8, 5J6-8. The number "5" in those last numbers is a code that refers to the aux driver board. If you look at the schematics, you'll see the purpose of this is for the coil circuit to pickup a flyback diode. Coil diodes were removed from the playfield coils and given a place to live on the aux driver board to improve reliability.

As I said earlier, grounding the tab tests the solenoid circuit downstream from the MPU driving transistor. Since it didn't work, the following possibilities:
a bad connection anywhere along the output path from MPU Q9 to the pf GI relay (MPU out, In and out of aux driver, and playfield connectors)
failed relay
failed flyback diode.

From here you can rule out the relay (yes, you replaced it...but ground the trigger wire at the relay board to be certain it works.)
Check connectors 1P12-2, 8P3-10, 5J2-8, 5J6-8
Check diode D10