I posted my original frustrating day at the dealership, but wanted to offer the final story. After the original 8-5:30 day at the dealer they could not get the steering to "zero out" so I had to leave it and drive home a loaner car. They ordered a whole new rack and pinion steering gear assembly (which took about 2 weeks) and installed that...it would not zero our. So, they ordered ANOTHER BRAND NEW steering gear assembly and steering ECU and replaced both and finally, 30 days later, it finally zeroed out.
All this on a car that had never had any steering problems of any kind. They told me mine was the first they did, and that they've done a dozen others with no problem, but that they have one other Prius going through the same sries of replacing the whole steering system twice and the ECU.
At least it seems to be working fine, like it did before I took it.