This feels like a common courtesy I don’t see enough, particularly the second point. In a conversation circle, always try to keep your side vision open to people trying to find their way in. It can be awkward to try and find your own way in so giving them a spot tells them they’re included then drop a quick brief on the conversation “We’re talking about pineapples on pizza, good or bad” gives them context to jump in immediately.
I’d also like to add: read the situation and adjust your etiquette accordingly. For example, I personally think it’s pretty damn rude to interrupt someone. However, with some groups, interruptions are just an expected part of the flow of conversation. Try not to be weirded out by harmless etiquette differences that others seem to be taking as normal.
Relax. You’re not the Manners Marshal. Have fun!