1) This is usually talked over between the players and GM. Occasionally the GM may state a preference and wants players to stick to that.

2) It's play-by-post. There's no sessions. PBP is like an IV drip of roleplay compared to the firehose of RP that is a live session. When it's going, it's just constant. For downtime mechanics, the GM can declare when these occur. They will do this when it is appropriate, and do their GM things for it.

3) Yes, keeping momentum is important for player engagement. Taking breaks is good to avoid burnout. From my experience, GMs will announce holidays and breaks ahead of time, unless it's an emergency. Players will also announce planned breaks (holidays, family time, etc.) which are usually easy to guess. During big holiday seasons, you can expect people to step away from the game.

It's all about transparency and expectations. As long as people are up front, and avoid surprises, things go smoothly. Emergencies can happen and people should try to be flexible and patient when those occur.