Few ideas to consider:
1) Drop democratic part of new coin. Decentralized currency is democratic by design. Majority of users can ALWAYS change how protocol work (eg. change interes rate) by just realeasing and installing modified source. There is no need for built in mechanism to do it.
2) Drop initial mining and
make distribution of all coins in genesis block. It can be nicely done using bitcoin blockchain. I can post details of my idea if you are interested.
3) If you adopt 2) you don't have problems with initial stages of money distribution so you can focus on making most efficient PoS system. Efficient transaction confirmation is most important part of new coin. Bitcoin don't have problem with 51% attacks it has problem with extremely expensive mining.
4) You said you want to make PoS as simple as it can be. You use 2 different kind of blocks. Why don't simply make system similiar to bitcoin with one modification: person with twice as many coins need to make half of work to validate a block?
5) Make something with ever growing blockchain. I like the idea of making a balance sheet every N blocks.
See this thread