I'm going to side with the OP. When someone has insulted your wedding plans and called you names its not unusual to turn around an use derogatory names when describing them.
It may not be unusual, but it is also not polite.
The OP tried to be nice to Suzanne (maybe Suzanne thought she was condescending), but Suzanne knew that calling someone names was rude.
So did the LW. Two rudes, last I checked, do not make a polite. It doesn't really make a difference if the LW's name-calling wasn't done to Suzanne's face.
As for the bridal shower, some people do think its very tacky to invite all the women to the shower. Old school etiquette says you should only invite a small number of guests. If that is what the OP is used to I could she why she would be surprised to be invited to give yet another gift to someone who obviously dislikes her intensely. Even $5 would be too much for me in that case.
I don't think there is any indication, at least from what we know, that Suzanne invited all the female wedding guests to her bridal shower. In fact, I think there's pretty clear evidence that she didn't
invite all and sundry:
After some digging, I found out that not every woman was invited to the shower, but that she decided to include me because I had given a nice engagement gift.
Given that the OP found this out by "digging" (i.e. hearsay), I'm not completely sure that I trust her assessment that she was invited because of her nice engagement gift. Or at least, I don't trust her 100% on that. It's actually far more likely that Suzanne included the LW because she was the best man's fiancee. As someone else already pointed out, the guest list would have been made up far in advance of the email exchange the LW mentions. Given that most mail takes about 2 business days, chances are good the invitations were even mailed before that particular email exchange.
Basically, I think there's plenty of rude behavior to go around here. Neither Suzanne nor the LW come out smelling like roses, but I don't think that the LW deserves more benefit of the doubt than anyone is prepared to give to Suzanne. After all, this whole story is from the LW's perspective - I don't think it's incorrect or unfair at all to assume that she is painting herself in the best possible light (and thus, painting Suzanne in the worst).