I'm typing fast as I have a lot of people to answer to, it would seem.
That constitutional republic you're speaking about is a democracy.Constitutional republics put checks on the power of any one branch of government to dominate, and guarantee the protection of rights. Democracy may or may not do that.
Hence Democracy is the most moral form of regimes.
You are arguing about why constitutional republic is the most moral sub-form of Democracy (which is a different argument).