I've done several mini-missions. That's missions of less than a thousand words where the agents tackle just one problem in an other wise good fic or a short badfic. Miah once wrote a mission where her agents were sent into six badfics to sort them out.

I'd say either works.

I don't quite understand what you mean by "found on a blog where someone other than the original author reviews it".

Well, of course someone other than the original author reviews it. It doesn't count as a review if the author reviews it.

But did you find the badfic through that blog or in that blog? Don't think it really matters. Fanfic is fanfic no matter where it is found. I've missioned at least one story I first found through a blog/that was in a blog (LJ).

MSTed versions count as secondary sources. If it's MSTed it's likely it's sporkable. But it would be best and try to find the original source. Though I believe it is customary among MST'ers to MST the whole story, you should always check if they haven't left anything out to make the MST more compact.