If I may, after running a good half dozen semi-long lived games, drop outs are inevitable. I think it would be best to carry on like the dropped player characters never existed and if replacements are found, introduce them whenever is possible. There's numerous reasons a person wouldn't arrive for the meeting, and it's a big world. Who says we don't meet these people well into the adventure? 

Nothing kills a game faster than getting hung up on one or more people who aren't participating. Always cater to those who are active and be ready to drop characters at the drop of a hat. A few inconvenient narrative issues are insignifcant to waiting weeks between posts, because that's when people start to lose interest.

My own character's situation, for instance. Two players are active and trying to move forward but can't because the third probably dropped out. That's three players out of action because of one disinterested party.

Long story short, keep pressing forward. If people aren't checking in of their own will, put their characters in a purgatory bin if they miss the posting timeframe and post when they have to post by. If they miss it, cut them loose. No sense in a few dedicated players getting neglectrd because of an idea you have to have a full roster for the story to work.