Not that I don't agree with your assertion of scrutiny, but...
1) If you watched any of the videos shot by foreigners and Iranians alike, you'd notice that there were at least 50 across if not more. This makes your calculation wrong by an order of magnitude. The videos are there to prove it.
2) Mousavi being under house arrest and then let out due to pressure caused by so many people marching and protesting is a more likely scenario. Don't underestimate the stupidity of snap decisions made when the reaction was unexpected.
3) The president of the election committee can dissent with the Ayatollah, even if it were dangerous for him personally to do so. Also, Ayatollah went back on his proclamation a day later and asked for an inquiry into the elections. This makes that the president's claim that much more valid.
None of these claims are unlikely, but caution must be exercised anyway. I'd rather believe the people in the streets than what someone from the media can tell me. If you don't believe me, watch the Twitter streams. There's a lot of misinformation there, but also a lot of good information. Sifting through this is a journalist's job.