GFCI is working
Some are overly sensitive, which can trip because of switches or relays, as your have seen with washer.
LESs are draw very little amperage. Tripping breaker indicates a bad bulb or wiring problem.
If I am reading right, when only 2 strings were connected, the problem went away.
How many strings are you using. Your problem is with one of these. Plug in one at a time and see which one trips the breaker.
I am guessing, he was checking Ohms with his multimeter. It should read 0.
Reading 40 indicates crossed wires.
After 36 years in a related field ,and engineer is the last guy I would listen to......just sayingif that’s the case, wouldn’t it cause the breaker to flip, even after they are split up?
My neighbor is an electrical engineer and he thinks it’s the lights also, and that maybe a hot and neutral are flipped or touching etc
After 36 years in a related field ,and engineer is the last guy I would listen to......just saying