But I can always point you out to Das Sporking. I especially recommend you the Fifty and Twilight sporkings, since they also include canon characters being... drafted for the sporking. It should give you a pretty good idea of how this is done.

...I don't know how to find MSTs that aren't already MSTing themselves, as one person put it.

... is that they're really easy to cowrite.

They consist of every line of a story, split up for comments where the writers want to split them. So open up a Google Doc, copy the text of the story in, and then... start commenting on any lines that you think deserve a comment.

You don't need to be on at the same time. If you want to reply to something your coauthor said - reply to it! And then reply to their other comment three paragraphs down, and then make several of your own on uncommented lines, and then go back and add another reply somewhere earlier.

MST writing is - or can be - amazingly non-linear. It is tailor-made for timezone difficulties.

hS

1: Make some new characters. An MST is not exactly a PPC exclusive thing, so you have some additional choice here, too.

2: Get someone to write sections of it without you, and leave notes for them at the end. Repeat for you. If you have phone access, you could send them Skype messages or something as well. Don't really know your timezone, so I'm not sure if the last one could work.

3: Yeah, that's a more common problem than some people think. I never stop tweaking any canons I write, since they never seem quite right. Fortunately, having a bunch of collaborative editors around can help you here. I find that canons end up feeling a lot more well done once you have other people tweak them. It gives you free advertising, as well, since the people you have editing will likely spread word of the MST, getting you more readers quickly.

Hope this helps.

1) You don't necessarily have to MST with PPC-related characters. If you want to, you can always use it as a method to develop your less-developed ones. If not, well, you can do anything from just snarking at the fic like Astral Void did when she sporked 50 Shades of Grey (sporking of NSFW squicky material) to writing completely new characters for the task.
2) Well, what seemed to work for me and SeaTurtle for "Grey Area" was that I'd write a line (paragraph, whatnot), say "tag" in a comment or whatever, then he'd do the same. Rinse, repeat.
3) Can't help you there; I don't think I ever wrote a canon character (in an RP — didn't make any attempt at a fic) in a way that felt OK to me.