Muffler rotting, I have some parts but some of the joints are welded, should I buy a CC or just get the new parts welded on?

Car/Location: 2010 Nissan Sentra SR, 2.0L Engine; Massachusetts, USA

Background

7 years ago, my car started making a noise. I brought it somewhere, the guy said there was a hole in the muffler, and he welded the hole. A few years later, another noise, more welding by different guy (I had moved by then). I’ve done some other work on my car (struts, brakes) so after some online research I decided it would be easier to just buy the muffler pieces and replace it all instead of dealing with this every other year when my car gets inspected for emissions. What could go wrong?

I bought the parts (resonator + muffler + misc mounting pieces) without actually looking at the underside of the car, which turned out to be a poor decision. Once I started trying to do the work, I noticed that all the welds were at the joints, so you can’t replace the muffler without re-welding it on there. Here’s a diagram:

    +---------+            +---------+              +-------+
----|Catalytic|-----||-----|Resonator|---+       .--|Muffler|=
    |Converter|    Weld    +---------+   +--||--/   +-------+
    +---------+                            Weld 

Last year a mechanic inspecting my car mentioned that the muffler was starting to rot out. I mentioned my parts purchase, and he offered to install the parts I had bought. I brought it in, and of course he noticed that it needed to be welded. He wanted me to buy the next piece after the resonator (catalytic converter) so that it wouldn’t need to be welded, claiming that it didn’t make sense to replace everything and have that one weld in the system. While I agreed, after researching the emissions-related issues for converters I got confused and didn’t take any action.

My father in law knows a guy who said (without looking at it) he could do the install, including welding.

Dilemma

My car needs to be inspected this month, the muffler is being loud again, pretty sure it needs to be addressed this year. I still have the parts. What should I do?

• Buy a catalytic converter (this seems complicated due to emissions rules) and try once again to do the work myself, or have the mechanic do it.
• Have someone install all the pieces and weld the single joint between the resonator and the catalytic converter
• Go through with the inspection and have them weld it again if it fails
• Other?