The person making the RP basically can do whatever they please, there isn't really a set of solid rules you need to worry about (although there are some excellent guides to be found here: http://www.roleplayerguild.com/topics/2748/posts/ooc ).
The best advice I can offer is pay attention to games that have been around a while and see how the GM in that game runs things, how they format their OOC, how they select players, how they keep the plot moving and satisfying.
As for your specific questions, there isn't always a solid posting order, but good manners dictates that every player should be given the opportunity to post again before the original poster continues with his or her next post. This of course is fluid; it's suicidal for a game to wait up on one person who is missing for over a week without any indication of where they are, and a GM always should keep things moving. Knowing when to do that comes with experience.
If somebody flakes, just move on with the people who stay. I like to accept more than I want for a game because I assume I'll have a few people drop, and if they leave, you can brush their characters out of the story in pretty easy order and anyone remaining is a solid core you can do great things with. Treat loyal players well and they'll do the same. The thing that kills a roleplay off fast is if neither the players or GM have faith or patience in one another; respect that people aren't machines who can post every day and keep them involved, and they'll likely reward you with a long-lived and entertaining game.
