(1) yes, the "1usd worth of STEEM" is just a means to give confidence to the people, but i believe the ultimate goal of the SBD is to have a parity with $USD, as descript in White Paper(pg15)
![](https://www.steemimg.com/images/2016/08/04/parity92e60.png)

hence, when "1usd worth of STEEM" is not enough the derived the desire 1:1 peg, witnesses should try to gradually increase the amount of collateral, until a new balances is found.

(3)  as mention in the post, this is the Impossible-Trinity problem. 
If our priority is to have a fixed exchange rate, then we shouldn't stand still with the fixed monetary policy (interest rate)... if we want more control on the interest rate, then we will never had a stable SBD.

Totally agree with your point that the interest rate should be discussed and vote. but this should be done periodically, and adjusted from time to time according to the market environment...  When is the last time they have a meeting or discussion?