* 3 million people protested in Tehran: I was just pointing this out to someone. If you truly had 3 million people and they stood in the road you'd have 5 people across. Take those 5 rows and have everyone line up behind them. In that case 3 million people would go on for roughly 114 miles (assuming about a ft of depth per person). It would be visible from space.
More important, a few thousand Iranian police wouldn't stand a chance against a group 3 million strong. They could overthrow the capital with that many people.
* Mousavi under house arrest: Possible but pretty stupid. This is the most plausible of the ones listed but it would be suicide for the ruling party to do so it stands to logic they wouldn't.
* the president of the election monitoring committee declared the election invalid on Saturday: If you've watched the news for any amount of time you know the election committee can't invalidate an election the Ayatollah has approved of.
So under basic scrutiny all these claims seem unlikely. If people are believing them on face value they clearly aren't putting much thought into what they believe and that's the problem.