A few ex-mechanic friends of mine have gone in to HVAC install and repair. One had a good experience of car A/C so had a good understanding of how systems work.

A stock exhaust may be cheap on ebay or similar

Absolutely, they’ve also recommended other work before. the only reason why I went there is I had an extended warranty when I bought it from them that requires them to service it, but that is due to end before the next service.

Many modern cars have a 2year/20,000 mile service interval. I wouldn’t worry about it.

I’ve had a dealer recommend me work (wipers, belt, thermostat and coolant change) less than 3 months since they changed it. You’d think they’d have a quick look at their records before writing up a work list