I don't know if the white paper says so but it is clear to me and Dan has also said that interest rate changes are a long-term factor and don't really have much influence on short term supply and demand. Given that SBD has been between -25% and +50% of par in the relatively recent past I don't think much can said about what the interest rate should be. That's just, like, my opinion of course.
Directly placing market orders
As I said steemit is a market participant. I don't see how that is questionable. If they were push the price away from par, you would have a point about it being "manipulation", but they are doing exactly what rational market participants would do if they find SBD selling at a discount and believe that the pegging mechanism works over time: They are buying it.