I guess the engine coil replacement is worth a shot. I don't know why the mechanic didn't do it.
Here's the calculus I have always done. I've done it with cars, and used to do it with computers back in the day when they were actually fixable and not cheap and disposable.
What is it worth right now, as-is, with full disclosure to any potential buyer?
What would it take to fix it? In your case, how much for the ignition coils?
What would it then be worth after repair, but otherwise as-is?
If the value increase is equal or more than the cost of repair, then fix it.
In your case, that's going to be a tough calculation with the million miles you have on the car. It's not going to be worth much no matter how well it runs. May or may not be worth fixing it. It also may boil down to one of those situations where it starts and runs fine every day, get's you where you want to go, and other than the light being on does just fine. That light being on may be irritating, but it doesn't hurt anything and over time you'll learn to ignore it. Mine's been on for over a year.
FWIW, I'd get a second opinion from an outfit that knows how to do a proper electronic diagnosis. But from what you've posted I think the coils are a good bet for being the culprit.