Friend, you (or your mechanic) are making a mountain of a molehill!
I believe
this is the cat you need. It's $208.25 used.
Make sure the rear cat does not have any debris from the front cat blocking flow.
BTW, if your getting a CEL, that points to the primary cat as the secondary is unmonitored and will not generate a CEL.
It sounds like you mechanic recognizes that this is not your area of expertise and is not going out of his way to educate you...
This is a 2.5 hour DIY at best. This is a situation where you apologize to your significant other for because you have to blow a Sat morning turning wrenches...not a "I guess I'll just sell it" situation.
