After making temporary repairs to the Volvo's exhaust system over the weekend to replace rusted-out hangers and keep it from falling completely off, I got it into a muffler shop yesterday and got a chance to take a proper look at the exhaust system up on their lift.
In the vernacular, "OMG." The previous repair was WAY worse bodged than I realized. The turbo muffler apparently rusted out, and was replaced by cutting pipe and welding in an aftermarket resonator a different size with different-sized pipes, and to make matters worse the idiots butt-welded pipes more than a quarter-inch different in diameter without even overlapping them. Between that bodged job and general exhaust-system corrosion, it needs the entire exhaust system aft of the catalytic converter replaced. I was hoping a new pipe section could be welded in, but having gotten a good look at the pipes now, they're not weldable ... the remaining metal is almost tissue-paper thin in places. Try to weld it and you'd just blow more and bigger holes in it.
The guy at the muffler shop, which I will not name for his protection against possible corporate retribution, said "If I order the replacement parts for you, I have to use approved sources, and you'll be out a thousand bucks for parts. So what you do is order your parts online, then bring them here, and I'll charge you an hour's labor to put it all together for you." Now that's an upstanding guy. :)
I just got a notice that the first installment of parts shipped (it's coming from two different warehouses). We're saving $350 by buying the parts online, and still getting OEM parts for our money on everything except the muffler.