Interesting case. I would be wary of lawyers who make big promises in this case. There have been plenty of articles about just how much money these sororities and fraternities have to throw around during legal disputes. You could end up spending and spending and spending just to get out of this situation which IMO is not ideal. A few hundred bucks for a few crappy t-shirts? Sometimes it's best to know when you hold em' and when to fold em'.
Second, I know reddit and this board in particular gets off on justice boners and seeing people "get their come uppance" but please keep a few things in mind (devils advocate stuff. I'm not an expert but just my personal experience here)
Sororities and their extensive networks of alumni can actively discriminate against people as easily as they can actively help them. Be prepared that in the future you may run into someone who has an issue with you because of how you left the sorority. If you leave by pissing everyone off, you will experience the phenomenon like the phonecall with their bank service. I have seen it happen to friends.
Having had some experience with the greek scene, I know how much everyone in it gets off on power. People leaving the scene also get all high and mighty and want to talk shit about how awful it is and how terrible the people are. People who stay in it talk shit on the people who left saying 'oh they couldn't handle it' blah blah blah.
I advise you to take the middle road which is be as respectful and professional as you can throughout the entire process. I can't tell if you are or you aren't. Honestly, some of the e-mails and conversations you're describing feel like everyone is taking it a litttttlleee too personally.
Hope my words are taken in the spirit I intended them: helpful, and concerned. Not judgy and assholish.
Good luck with everything.