[mcfe]

I guess we just need more information. It is true that a slipping clutch would rev higher, but I am guessing there is a point where it's not that bad but also not that good.

However yeah the idle problem also suggests some other things. He's replaced most of the things that could cause it.

Resetting the ECM will just reset adaptive values which can easily be done with the battery disconnected for a few minutes. Beyond that if the car was tuned such as to increase horsepower, we can't say what the tuner did. But if it ran fine for months with a tune I can't imagine it being the problem. And to reset the ECM to factory settings it would need to be reflashed.

There are 3 more things I can think of, that need to be tested for before suspecting the ECM has gone bad.

Exhaust backpressure from a clogged catalytic converter. This should manifest as the inability to rev very high , again if not that bad it could potentially cause this in my opinion. But imagine him coming to a dead stop with the turbo spooled up, he goes neutral and all that built up pressure is pushing back causing the car to not be able to maintain 750 rpm idle speed.
Uphill would be a battle of the car trying to rev higher but can't from the backpressure.

Boost pressure sensor aka MAP sensor, I didn't see this being replaced or tested during acceleration what it's values are. Maybe there is boost loss, but I would imagine there would be some codes.

Faulty Oxygen sensor, if the fuel mixture isn't right it could cause jerkyness as more fuel or less is injected in the engine.

A quick stop at an intersection involves pressing on the brakes. The brake booster needs vacuum to operate. The seals on the booster can fail. Another thing is the check valve of the brake booster.
Some volvos were also equipped with a vacuum pump for the brake booster. Perhaps the hoses there could also be checked?

Has the compression of the engine been measured? It's fairly easy to do.