In the interest of evaluating this, let's be more specific. This scenario would require either of FIU or UALR to beat FAU in the finals. With FIU and UALR being on the side of the bracket with the home team and South Alabama (and playing the home team and South Alabama respectively in the opening round), I find this extremely unlikely.
Even if the near impossible scenario above plays out, the NCAA is not going to put an arbitrary ceiling on the number of teams awarded postseason play from the Sun Belt (the #5 RPI conference in the country). The Cajuns will get in (or not) on their own merits. I would only be modestly concerned if their RPI dropped to about #40. I am not concerned about #30->#35.
Another silver lining in your above doomsday concoction is that if FIU or UALR were to win the Sun Belt Tournament over FAU (with the Cajuns going 0-2), there are better odds that this would not happen without the Cajuns having the benefit of losing that second tournament game to South Alabama ... which would have minimal impact to the Cajuns' RPI.
Meanwhile, the Cajuns' record vs. the Top 50 RPI is a very respectable 5-5 and 7-6 vs. the RPI Top 75.
When have you seen the Cajuns get left out with an RPI around #30? I think your doubts are getting the best of you. It has never happened.
Brian