The template for her being a disaster in the making is, of course, Thérèse's wedding. She loves to play the innocent but let's face the fact that Mike was dead on about how the real reason she bothered going was to make a big, ugly graceless spectacle of herself in order to stick her tongue out at someone for the very stupid reason that she cannot stand the idea that a friend of hers can have a friend who is not her. She can scream and moan and belly-ache about how unreasonable and mean and heartless her sitting duck adversary is all she wants but the plain fact is that she can't stand being told where not to go or who she can't be around or that her presence isn't mandatory and she'll do anything to make people pay for the crime of denying her.
She also tends to downplay the damage she did at Shawna-Marie's wedding. Not only did she stand around whining about how Anthony brought along a co-worker as his plus-one, she ditched her escort so he could half-way die of alcohol poisoning in order to catch the eye of the only chance she had to impress her asshole parents. Lynn and the Pattersons might see all that as romantic but it's what I like to call 'beyond the pale'....especially since her reaction to finally being told what her browbeating the man into consuming beverages that would kill him is a flurry of arch victim-blaming that makes me think that her response to being told that she's big enough to take responsibility for her actions will always be to shriek "I LITTLE!!!!!!"
Fortunately, I have the perfect solution for anyone who wants to get married without Lizardbreath showing up and mucking it up for everyone: hold any future weddings more than an hours' drive from where she lives. Given that she's Elly 2.0 and sees having to travel to somewhere at someone else's behest as being asked to crawl on her knees like a defeated supplicant, the ceremonies might as well be taking place on the Moon.