Wow. I wouldn't let that shop near a Volvo. For one, there is no "valve cover gasket", per se. The head is split into two pieces which sandich the cams. Oil on top of the motor is a result of oil being pushed out of the oil filler. Causes are a brittle oil cap gasket, or, more likely, excessive crankcase pressure due to a restricted breather system.

I have only seen one bad injector on one Volvo in 12 years of working on them everyday. Coil, on the other hand, is quite common on Volvos with the coil-on-plug setup this S60 has. A distinct miss - rough idle, could be a clue.

Generic scanners do very little for Volvos and can send you in the wrong direction. Need to extract Volvo specific codes, lest you chase your tail like you did with the AMM. Find an indy or dealership. Modern Volvos are not for the average Joe-mechanic do go futzing with, unfortunately. They are quirky and specific. Folks ask me to work on other makes and I decline because I'm not familiar with them.

Throttle bodies can be cleaned and I have had success. Proper codes and diag are critical, however. Good luck.