might sound ridiculous, but have you checked the non working globes?

that interconnect board looks fine.

is all the GI in the backbox working?
is the coin door lights working?

have you checked the fuses out of the board with a multimeter for continuity?
check the tension of the fuse holder clips and for any tarnish.

at 7J3 on the backbox interconnect board do get approximately 6 VAC across pins 1&4, noting that pins 1&2 are joined together as are pins 3&4, this is your AC input from the transformer.

from there check 2J9 pins 1&5 for about 6 VAC and pins 6&7 for about 6 VAC, these are the 2 strings for the GI to the playfield.

looking at the schematics the GI comes straight from the transformer as a yellow and a yellow/white wires then splits into 2 yellow and 2 yellow/white wires joining at the interconnect board in the backbox at 2J6 where it splits into 4 of each which are fused, then out at J7 (Backbox), J10 (door) & J9 (Playfield).

the playfield GI is split into 2 circuits at J9

Pin 1 (white/violet) & Pin 5 (violet) = 1 circuit
Pin 6 (yellow) & Pin 7 (white/yellow) = 1 circuit

in the previous posts I was starting at the transformer asking for readings then working along to find where the possible break in the circuit is.

the GI is AC, so without the plug connected at J9 what voltage readings do you get at J9 pins 1&5 also pins 6&7 ?

my understanding of the number prefix is a reference to a board or transformer, 7J3 being the transformer and 2J9 being the interconnect board.

the relay (A/C) is on the auxiliary power driver board for switching flashers and solenoids.

if you are 100% sure you have 6VAC on the 2 pairs of pins on the interconnect board then this is good news and the problem is from the plug to the playfield.

although IDC's (insulation displacement connectors) you have may look like the wire is secure in there, the insulation may be, but the actual wire may not be making contact with the pin itself.

I'm curious which side is out on your playfield? the white/violet & violet, or the white/yellow & yellow? You did write that you got 5.5VAC on the white/violet & violet at the playfield......are you 100% sure on this, just checking is all.

if you are sure, next confirm continuity from the connector to the playfield for each of the white/yellow and yellow with the plug removed. I don't imagine you have a spare header lying around so you could use a needle or a nail to put in the connector, acting as a header pin, and make sure with one probe on the now inserted (whatever you used) in the plug and where it next terminates on the playfield.

Lifting the playfield up and back you should be able to see where these 2 pairs of wires join at the playfield. You can also measure for 6VAC here on the pair that don't work, maybe also try the pair that does work just so you can get more acquainted and confident with what you are doing and what is and isn't happening.

Also are you using LED's for GI or original incandescent globes? I ask this because an incandescent globe will act as a short when reading for continuity as there is a closed circuit from the base to the body of the globe, but not so in a LED. Ignore the following you have LEDs installed. With the game off and your meter on continuity, if you put your probes on both the wires you will get a beep, so if you tracing the yellow wire say, with one probe at where it comes in at the playfield (alligator clip is great for this freeing up your hands) and the other on the next yellow in the row it will (should) give a beep, but if you touch where the white/yellow joins the globe you will also get a beep due to the incandescent globe, so, as annoying as it is, should it go this far, you need to remove all the globes in that white & yellow/white string. There may be a faulty globe holder here and you won't get the correct readings, I hope all that made sense?

Also, have you checked the non working globes/LEDs themselves in other working GI sockets?

my understanding of the schematics is that there isn't a relay involved with the GI.

well done and sorry to have lead you astray saying there wasn't, and I couldn't see a relay involved with the GI when it was mentioned in post 4.