May I still be held liable for a misworded section of a fraternity contract?

Well, you're right. It's a little unclear. But,still, it clearly means one of TWO things. Either, A) you need to contact them at least 24 hours before the event. or B) They have to give you an "acceptable excuse" within 24 hours. So, you'd have to do it sooner. Either way, your 3-4 hour notice violates it.