There is an immense disconnect between this statement and the litany of broken items listed in the OP that includes “including the power steering not working, and inoperative rear window, rear window defroster, and rear window windshield wiper. It sounded like it also needed a new muffler.”
It really sounds to me like several times a year you need to swing by the car washes that do the undercarriage.
Another vote for taking the $250 quickly before he changes his mind. I think you have been fortunate to have a reliable vehicle with minimal repairs for a decade and a half, whereby a bunch of expensive stuff has happened all at the same time - making the decision a very easy one.
What is the “immense disconnect”? All of these things popped up “relatively recently” like I said (i.e. in the last couple of months). Prior to this, the most expensive repair for the vehicle was replacing the shocks at 200K miles, which you could argue is a routine maintenance item.
The rear window (which rolls down), rear window defroster, and rear window windshield wiper all stopped working at the same time in March, so I assume they are related (like a wire harness issue and/or blown fuse).
The muffler is another routine replacement item – I’ve replaced it twice before over the last 240K miles.
The issue with the alternator just happened this week.
If not for the frame rotting out, I would have at least considered getting these things fixed. It would have been worth a couple of thousand dollars for me to have a working vehicle, and I could then have turned around and sold it and got some of my money back – certainly more than $250.
I did, but evidently not enough. The problem is that the corrosive road treatments get applied to the roads here in New England several times a week for most of the winter.
With that said, I do need to make a point of getting the undercarriage washed more frequently on our newer vehicles – like right now, now that winter is over.
Right, I have pretty much come to the same conclusion.
I told the mechanic yesterday I’d take his offer. I’m trying to track down the title now. 
I feel like I’m in mourning – which I know is stupid for a vehicle, but I can’t help it.
About 10 years ago I had a 93 Toyota pickup in great condition. I had just spent a bunch of money installing a new clutch, shocks, radiator and some smaller bits. My goal was to sell it to get a vehicle that could carry more people. I found an SUV I wanted to buy and just to see how much my truck was worth, asked for an estimate of the trade in value. The mechanic came into the saleman’s office about 30 minutes later to give me the news. Trade in value, $300. He said the truck runs and drives like new. But, and it was a big but, the frame was rotted at the transmission cross member and had many other spots with heavy rust.
My first thought was why didn’t the mechanic that replaced the clutch and shocks find this. The salesman bumped the trade in to $600 so I took it. Drove away a bit later in a used 2004 Trailblazer that 8 days later puked the transmission all over a freeway.