I called US Customer Service to try to find out how they handle the seating on Hi Fly.
1. She tried to tell me that they don't plan to use the First Class seats so that everyone gets treated equally. I said that's not true. People have already been seated in them. She recanted and said they would be used only if there aren't enough seats in Business Class.
Your original assigned seat with have no impact on where you are seated on HiFly.
2. She confirmed that although she knows the configuration of the HiFly plane, she can not assign seats. She can, however, take requests and forward them to the airport.
3. Finally, I asked about numbers and she said they wouldn't book more people than they could seat in Business Class.
Given how old and awful this plane is, I asked if there is any possibility that they might use another of Hi Fly's more desirable aircraft and she said this one best suits their needs.