With the dipstick in place, the engine idling, do you have suction or pressure when you undo the oil filler cap? A rubber/nitrile glove held over the filler is the easiest way to confirm.
It may just need some run time after the PCV replacement, a nice long journey once you've sorted the boost issue, and another oil change for good measure. But...and there is a but...if the garage have neglected to properly clear the lower port which returns the oil from the catchcan back into the sump you may issues down the line. To understand what I mean have a read through the following threads
https://www.volvoforums.org.uk/showthread.php?t=181833
https://www.volvoforums.org.uk/showthread.php?t=175153