If the fluid wasn't changed, it's likely that your dealer's correct and the problem may not occur until the trans gets to operating temp (hot), not just a brief drive around the block. Take it for a longer drive and see but don't go too far or the walk back might not be fun. Sometimes too, I've seen where after the trans slips so bad that it's not driveable (car doesn't move at all), letting the trans cool down for a few minutes (engine off) brings it back to life enough to get you home.
The PNP switch is often guessed at here but that's simply not always the only thing that fails with those trannys, although it is a common problem but with different codes, symptoms.
An Indy shop will buy the same reman trans from the dealer (no other source really) and likely mark the price up. It's about $1,600 for the OEM trans at the dealer and book time is 10 hours labor. An indy's labor amount might be less but I'd want to be sure that they have actually experience with replacing FWD Volvo trannies first. Whoever you choose to do it, request that the rear main engine seal is also replaced at that time so that you're not paying another 10 hours labor later up the road.