Hi @ebryans - thanks for taking the time to put all these thoughts together. A few things in reply.

1. Unfortunately there is no way to implement this where it will not be abused. If users get more power for having multiple accounts, they will split their one big account into several little ones.
2. Implementing this at the blockchain level has it's challanges, but many users are already doing this type of transaction "off chain". One thing to keep in mind though is that if a user buy SP directly from a user at a 25% discount, that is still taking buying pressure (demand) away from liquid STEEM - which puts a downward pressure on the price. If you assume that the same user would have bought liquid STEEM if the price dropped 25%, then this demonstrates how the demand is gone. One alternative that has been discussed is the idea of allowing users with SP to power down instantly at a 25% (or higher) rate, which would basically be paying the network by reducing the supply.
3. I disagree. Having witnesses who have a major stake in the platform is generally good, because you want the witnesses to care about the success of the platform. In the few scenarios where there are decisions that have to be made that are not in the best interest of 'stakeholders', regardless of whether a witness themselves holds a lot of stake - it is important to keep the interests of the major stakeholders in mind. To disregard the interests of the major stakeholders (regardless of how they earned their stake) sets a very bad precedent. Why would any future investors want to buy into the platform if their interests will be disregarded 'for the good of the platform'?
4. It is not possible to do this with the technology that Steemit is built on. Multiple accounts are going to be a reality that we have to accept and adapt to. There is no way to prevent it.
5. I agree.