On adding GI - I normally don't use LEDS on my EMs except in special cases. But on your four side pop bumbers, or all of them, perhaps you could try 5SMD warm white LEDs where one LED is on top but it's surrounded by four smaller ones that shine sideways, which would light the perimeter of the pop bases. Comet's are called 'insane bright'. That would help light the playfield more, hopefully without making the caps overly bright. If that was the case you could try just blacking out the top LED. I've never tried them so I don't know, just a thought. I'm not even sure they would fit under a cap or how the top LED would look on the cap, might just make a spot and need to be diffused with something.

I thought it might be a concern with the lamp wires interfering/shorting with the switch blades since the hole is pretty small but might work as long as the wires are well insulated. Another type LED that might work are the 1SMD 'flex' type with the longer leads, if bright enough. The LED itself could lay flat with the two wires coming up behind the switch and they would shine against the underside of the guides and diffuse onto the playfield. Maybe add sockets under the playfield, or just staple the LED wires under there and lengthen the leads if necessary.