There seems like a few different issues going on.
1. Broken Spoke.
2. That idiots behaviour.
3. I don't see the lights as an issue. More a smoke screen to avoid issue 1.
Broken spokes are rare but they do happen.
Aside--you can still ride the bike with a broken spoke. ( I have had 4 brake before I fixed them).
I would judge that a bike that old shouldn't brake any spokes.
A spoke is about $1-ish to replace.
And a broken spoke doesn't necessarily mean that the rest of the wheel will break. Bike shop guy should have explained this to you.
Best bet is to replace said broken spoke, check the tension in the rest of the spokes and be on your way.
This is what I think you should push with fair trade.
It is unfortunate that the guy is an idiot, but that is a different issue that needs addressing seperately.
Make sure it is documented that the lights have never effected your other two bikes and for how long.
Unfortunately it seems like a massive issue for a simple fix.
I wouldn't trust the guy to do a proper job fixing it either, given his effable character.
Let me know how it goes.
I was thinking more about this and that the broken spoke might be seen as normal wear and tear. Hence not covered under warranty. However, bike shop tosser could have offered to charge parts and free labour, a quick 10min job and you are out of his hair and likely to come back again and again because they look after you.
And also, I agree with everything hangdog said.