If they didn't charge you more labor the other times you've returned the car to the same dealership it's apparent to me that they're admitting fault. Oil leaking externally from a turbo wastegate (not unusual if it's seapage) can't cause the problems with what you're describing and there are no related sensors on that side of the engine either.

Now, if there's ANY holes in ANY of the turbo air hoses it'll do exactly what you're saying so they should just start there and not be telling you anything else that might be causing problems, like a waste gate actuator diaphram (which can easily be tested with a new tool that we in Volvo dealers use to test gas caps, a mandatory tool too BTW). It'll either hold pressure or it won't, no guess work is needed there. The turbo is also a very simple device and there's no need to speculate on it either, the turbine shaft just spins freely without hitting the sides of the housing or it doesn't. If it has a bad turbo seal (only 2, front and rear), the oil leakage that'll cause will be obvious too (no need to guess).

Labor to replace any turbo air hose should be very minimal as it only takes a few minutes to replace any of them. The most expensive hose runs a little over $100 so there's no way you should see a big bill for that.

If there's another dealer (or shop where they actually know Volvos) in your area, I'd get a second opinion before spending more money.