No one called Musharraf's coup "constitutional.".
You are dodging the issue: Ron Paul speaks of "overthrowing" Mossadegh as if the US directly moved to end a constitutionally authorized Iranian government, when in reality Mossadegh was a Marxist dictator.
The question is: why does Ron Paul have a soft spot in his heart for a deposed Marxist dictator of Iran and a soft spot in his heart for the current unelected Iranian regime?
You're missing the point. You're operating under the assumption the US government has the right to depose someone if need be. And Rep. Paul is operating under the correct belief if we have not been attacked by that nation, there's no need. It's not a 'soft spot' it's just none of our business.