Those throttle units aren't really serviceable so I don't know what exactly was damaged, taken apart and dryed out before. I also don't know why you'ld say that your dealer isn't customer orientated unless you've had a bad experience with one bad service writer? Most dealers, especially these days are very customer orientated as that's exactly why some are successfull. They're also scored by Volvo for CSI so they're constantly being watched and have to be as cordial as humanly possible to everyone at all times, a far cry from how things were 25 yrs ago from what I've seen. That said, I'd advise skipping the middle man and just take your car to a nearby dealer yourself, have them check the codes, etc. They won't charge you any more than they would your indy guy you're just wasting his time IMO.
If the throttle unit has "internal faults" stored in memory as most do when one sees that ETS light, it'll likely need replacing and they're not cheap. They're about $600 plus software and labor if no longer under warranty.
For future referance, most techs and people with newer car experience do NOT ever advise pressure washing a newer engine, Volvo or otherwise. There's just too many electronic devices that don't get along with water these days. A newer engine also shouldn't be covered with oil and grunge so there's no need to wash it.