RE: Yes, you DO earn interest on Steem Power. All (well, most) is revealed.

[preparedwombat]

A few weeks later, I did a follow-up post that tied up some loose ends:
https://steemit.com/steemit/@preparedwombat/how-much-interest-does-your-steem-power-earn-i-finally-figure-it-out

I might have to revisit this topic.