It's all done in this subsection of the forum delivering messages and responses in the thread. There are no set rules on what one can or can't do, as long as you stick with the setting.
The only thing one are to avoid is "power-play" E.I writing what another character does.
Example: Rob being my charcter and the innocent wanderer being Wolfsbane
Rob the Robber flew down from the tree towards the innocent wanderer with the intent of taking everything he had. Has he landed he hit the wanderer in the head, rendering him unconscious.
The power play here is "telling the story" as if I hit Wolfsbane and actually render him unvonscious. That is for him to decide, not me.
And how often one can participate doesn't really matter. You can jump in at anytime. The story can always continue, even if you leave in the middle. But the longer you wait the more you miss. But just write when you feel like it or have the time. Quality over quantity.
The thread creator himself decides how much he narrates, that usually depends on the setting and the creator. Is there a special quest, special enemies or pure sandbox( do whatever you want)?
Is the game at a standstill and needs a push forward? These things decide. The creator is the DM and is here to enhance your experience.
In this case Wolfsbane is also the Judge and Executor with the ability to remove posts and punish misbehavior. But don't worry about that, Arthgon is the only one that ever has been banned form participating.
Some of your questions might be answered in this ramble, otherwise you will have to make a numbered list with question.
also, I'm game for the "start as god, end has minion" thing.