Yes, go right ahead. Openspace is business application for internal conversations, as if you were in your office with your colleagues. But we don't see any reason why you can't use it for personal conversations.
Never. Openspace is designed to replicate an office environment. No conversations are recorded. No data gathered. Obviously Cafe sessions are open to anyone in your organisation who wishes to join in, just like you were chatting around the coffee machine.
We are here to chat, not judge a persons dress sense, haircut or background! When the option is there, it is socially considered rude to not turn on the camera. So removing the option frees people to concentrate on the problem being discussed not how they need a haircut... badly.