If there is an exhaust leak before the cat, will the computer detect it & send for a CEL? Also, whats a practical way to repair the flex pipe without having to replace it completely? A small bump in a drive way took a small tear at it.

 

There's no sensor to detect it as far as i know, the oxygen sensor sends information to the ecu regarding the exhaust gases and malfunctioning whenever it happens.
There's a product that you can find at auto stores to repair a small tear, but from personal experience, only small repairs... If the tear is bigger than 1 inch in diameter, you're better off taking it to an exhaust/muffler shop and see if he can weld a patch in there.