I don't shoot if I don't get paid. Simple as that.
Now MAYBE I would if there was really a good reason and I felt bad for them.
But I certainly would NOT put anything up online or give them any sort of work at all until paid.
Honestly, I wouldn't even do any post processing until I was paid, and if I was never paid, I'd probably just take a few of the best shots, process them, and toss them in my portfolio.
You said you have a contract, so you should have some legal ground.
Increase your retainer from $75 to at least $250, if not just say 50%. NON refundable, except if you can book a replacement for that date, or some horrible tragedy like B or G killed in an accident or something I'd give it back but other than that, keep it.
If you get half the money before you shoot, you likely will not have an issue getting the other half.
The reason I MIGHT shoot witout getting the final payment is because I'd already have 50% of the money, non-refundable. If I don't do any more work at all after that, then usually it's a decent amount for my time spent at the event, so if I got stiffed I could live with it. But then I've never had any problems getting paid.