As far as Airways are concerned, I would imagine they would open the application window right before the ORD job fair, or shortly thereafter. They probably won't take resumes, they never do, but they'll take your email down, either to check you off as showing up or to notify you when the app window is open. I would imagine that they would then update your app with having attended the ORD job fair. It can't hurt to show up.
I attended two job fairs for Spirit, haven't heard a peep from them. I'm hoping the third time is the charm, I'm not giving up on Spirit.
That being said, I got an interview with JetBlue, and I have to say it was a direct result of attending the WAI Job Fair, so I believe it's worth it.
As far as the career seminar is concerned, I'd definitely sit through it. A lot of it is common sense, BUT, someone had to have done the stupid stuff they talk about, so being reminded about the dumb things people do during interviews is immense. Plus, Aero Crew is offering good, smart advice on social media.
Good luck, I hope to meet some good APC people there, and hope you get an interview out of attending.
Cheers!