You can unroll enough seamless paper to cover the floor and provide a mini-cyc wall in the process. Unroll enough to extend say 2 meters out from the back wall, tape down the leading edge, then pull it back up just enough to put in the amount of infinity curve you want, and then clamp the roll so it can't unwind. Then you have a solution for your dirty floors and for your portrait backdrop.

As to the ceiling, since it is somewhat low, I would blacken it with paint as a first preference. If you can't do that, I'd get a big sheet of black fabric and tack it up. You can get black velvet backdrop material (terrific for its light absorbing properties) in roughly 3 meter by 5 meter size that would completely cover your sitting area.