2003 Chevy Malibu Electrical and/or Catalytic Converter issues?

We have a 2003 Chevy Malibu. It had a coolant leak and was starting to overheat. Turned out to be the intake manifold gasket (lower). Besides this, there were no other known problems except for suspension/shocks. Had someone from Craigslist work on it (huge mistake) and while changing the gaskets, there were no o-rings in the kit (so he said) to replace on the fuel injectors and they sprung a leak that resulted in a small car fire (he had husband start car with pool of fuel on the engine). The fire was put out with water, supposedly removed it all and cleaned up well afterwards. Mechanic then re-did entire job replacing gaskets again as well as plenum (said it warped), fuel injectors and part of the wire harness (because of fire) which all had come off another car with lower miles (I believe he said they came off a Chrysler with same engine) at a junk yard. Plugs and wires were also replaced since.

First day taking the car out it seemed to be running fine but while we were sitting in a parking spot with the car running, a fuse blew (the ac compressor), this made my temp and gas dash needles not work. Replaced the fuse and it kept blowing whenever the ac/heat or fan was turned on. Could not run ac anymore, unplugged ac compressor and fuse doesn't blow, fan and heat work. Temp runs line or 2 above half way mark continuously but doesn't overheat. Car started idling rough at stops (only when in drive), feels like it 'puffs' or sputters but is fine in park or neutral or when driving. Had engine light checked by autozone and they said it was PO420. Had cat replaced few years ago, but I was told at that time they did a lousy job welding it but it was supposedly re-welded from the shop that told me this. Headlights flicker dim to normal, and sometimes the automatic dash lights/parking lights don't turn on but still work when manually turning them on. Car is now rough starting too, having to hold key longer to catch and sometimes try more then once or have to hit the gas pedal so it doesn't stall on start. Took it to another shop and it of course wasn't acting up as much (probably because it was in park) and he checked my engine light and found nothing (check engine light goes on and off), he asked who is resetting the computer? Said I must have an electrical problem (which I had already assumed with all the other weird things happening) because it's not storing any codes either. Now, today the car was shifting hard from 1st to 2nd.

This is our only car right now, I really don't want to junk it after all the money we just put into it fixing the intake gasket problem. The engine seems strong and it was running good before any work was done except for the initial problem. I also don't want to spend too much getting it fixed as it is older and has almost 160000 miles on it. If anyone can give me some information on what could be the problem, I would be so grateful! I don't want to be taken advantage of with another Mechanic.

Thank you!