The wedding fayre post made me laugh too.
When working in the industry I was obviously on the vendors side of things and my job was to sell, but some do get really carried away (vendors and buyers).
I also attended a couple in the run up to my own wedding (which was way before I worked in the industry) But we were sensible and stuck to our budget, picked up a couple of small items and mainly used it as a way to get an idea of what we did and didn't want.
I agree op missed a golden opportunity to put bride straight. I wouldn't have made excuses, by that point there was no need and op clearly understood bride and groom in the wrong and COMPLETELY out of order and bride taking piss with what she was sayjng, and the relationship is buggered anyway! May as well be honest. Unlikely they'd have taken it on board but at least then op knows they've been as straight and clear as they could.
Sometimes a relationship is irretrievable.
I'm nc wth my sister for many, many reasons (lies, stirring, physical assault, running up debt in my name without my knowledge, being crappy to dd...) I've tried in many ways to sort things out, so have my parents in taking her to task over her behaviour but she'll never change.
Parents still in contact but mainly because she holds the dgc ransom basically - though as they've got older and able to see my parents independently they've gradually had reduced contact with her. They're well aware what she's like and take everything she says with a large pinch of salt (seriously the lying is pathological and over the most ridiculously unnecessary things often).
It's impossible to "hash it out" with people who completely refuse to acknowledge they've done anything wrong at all.
"Denial" is bandied about when people mean simply "too embarrassed to admit they're wrong" but for some it's a genuine mental block as part of their disordered thinking.
So at a certain point you have to say "I'm done" and just not have anything to do with them.
That's not a failure on your part it's self preservation and sensible.