You've read the White Paper closely and have taken the time to analyze and formulate good questions! I've been here a while but I'm only able to comment on a few points.

5 People who vote also get rewarded on an article. When they get paid out is that also 50% steem dollars and 50% SP?

Voting (curation) rewards are paid only in Steem Power, from a pool of 25% of the value of a post. Commenters are paid in Steem Dollars and Steem Power, same as post creators.

11 Can you start/stop "powering down" on a weekly basis?

You can cancel a power down at any time, even before the first actual weekly power down occurs. Last time I checked, here on steemit, when you choose to power down, you power down your entire SP balance. But I understand there is a command line tool available called CLI_WALLET that lets you power down any part of your SP balance.

As for sustainability, the idea is for Steem Dollars to become an accepted form of "money" similar to what is beginning to occur with Bitcoin, and there will be continuing demand for it as its utility increases. This can be aided by real world vendors accepting Steem and/or Steem Dollars as payment for goods and services, as well as by new apps being created as side chains on the steem blockchain which use Steem as their currency in some form.

That's my thoughts, and my opinion only. I hope others weigh in here.