The rail fare Amtrak is now charging for sleepers is the second "Value" coach bucket. I don't know when they changed it from the lowest "Value" bucket, but they did. They never used the coach "Saver" fare level for the rail fare for sleepers.

All passengers in sleepers pay the second "Value" bucket rail fare, whether on the initial reservation or added later on an Open Sleeper ticket.

And yes you can add a person using an Open Sleeper ticket to a room booked on points. I have done so in the past. Note it is not a change to the existing reservation, but a separate ticket. I don't know whether or not you can use points to book an Open Sleeper ticket. I used cash to buy the Open Sleeper ticket I used with my points reservation.

Using an Open Sleeper ticket will not change your original fare basis. If you try to modify your existing reservation rather than adding an Open Sleeper ticket you may well trigger a price change to current bucket.