I think the biggest problem with voting is that votes can be sold and there seems to be no way to stop this. That's a longterm problem with DPoS and not sure if there are great solutions. But several of your points would fix more immediate problems with the STEEM consensus IMO.

2 1 SP = 1 vote

This is the most important change. Once implemented, setting max votes would no longer be neccessary as every additional vote dilutes previous votes. Or you could allow users to allocate vests however they would like, with the rule that one vest can support at most one candidate.

I think this would make it substantially harder to make controversial changes to Steem. Because 30% of the community could elect witnesses to block a supermajority.

3 Increase the number of Consensus Witnesses

Agreed. Seems like we're wasting a lot of decentralization that is waiting to participate.

4 Expiration date for Witness votes

Possibly, but this would also encourage proxy voting to circumvent the expiration. And possibly other actions that result in users giving up security for convenience.

5 Introduce an incentive to increase voter turnout

This may be good, if paired with some mechanism to encourage intelligent voting.

6 Withdraw the voting rights of Exchanges with customer funds

I wonder whether there is a practical way to to accomplish this. You could blacklist known exchange accounts, but then it would just drive exchanges to use more secretive account structures. Also how do you define exchange? If I'm holding some STEEM for family members and helping them trade it am I an exchange? It will be hard to agree on a definition here.