These are mostly universal questions (they happen in every game system) So some examples
a) generally speaking, people tend to gather once a week or once a month (it can vary, but normally the most frequent is a week) when doing such games live. so long as everyone is on track, they do go very quickly, but at the same time they're pushing everything into that single day. Makes sense really, it's like going to watch a football game together. it's more fun in a large dose.
with pbp, if you don't want it slowing down as much, people will at least post a few times daily each. so long as everyone has clear posts about what they do, what's going on, questions are minimized to an extent, which means people can get straight to their action, roll, or whatever. Organized posting and making sure important information is posted help greatly.
b) I can usually pull off posting daily, but I'm comfortable with reading most of the time. I have seen games that will just ask for a single post in a day but encourage more, and others that are day focused. From what I have seen, people rarely have anything greater than (post at least once a week). Usually if they're that spread out, they try to get people focused on posting on particular times to encourage movement.
Naturally this is always a sticky point if a player (or especially a GM) go MIA.
c) Combat -does- slow down. it can't be helped. Ideally, there will be a set protocol for dealing with these situations. Examples include that when combat is known to be present, they like people to check on it frequently to post and avoid the slowdown (especially or the GM, since some rolls require them to be there in case a surprise happens). Some will have an assistant GM who can help keep the flow going so they don't have to show up as much, or more rarely, if players know what will happen, they can half narrate it themselves until it really is the GM's turn. Anything that helps smooth out the bumps will help keep combat from getting too slow. For example, minimize RP pose in favor of numbers, ans ave it for the glorious moments (unless you're a fast summarizer).
d) Three likely scenarios. One, they 'vanish' from existence to some extent and you go on without them. Two, things pause probably as the GM goes to hook one of the waiting players hiding in the lobby, possibly using 'One' to get combat out of the way to introduce the new guy (or they ghost the character to keep things leveled out). Or Three, things fall apart awkwardly, because a suitable replacement can't be found in the lobby or other drama pops up at the same time. Sometimes the missing player shows back up at the last minute, and promptly disappears again.
If you're lucky, they go MIA in a softer spot where its' easier to pause and find a fresh recruit, especially in games that work with hired guns or the like. Maybe they found a calling as a farmer? Maybe a farmer got a good called shot on them and they're not fertilizer? There can be a lot of ways to explain it.
Overall, as long as the GM is organized and prepared for the risks, that is a major help to keeping a game going. Players who are ready and willing to stick to a game, however, are almost as important - a GM cannot run a game if they have nothing; players can't do anything without a GM.
