Hello!
About pay with points only: It is not possible by default to enable payment providers only for purchasing points. But we can add this option as modification. The all installed payment providers are available for purchases by default. But credits module can work as you described in new UNA version.
About conversation rate: I am not sure that I understand your question. There are two rates options: one for purchasing points for money and another one to exchange points to money. You can set them both in studio panel.