Ben Bernanke Portfolio Balance Effect
My argument is that he is just looking to find a justification for a policy choice he made, and so there is no reason to believe that his arguments are completely coherent.

We need to understand why he is making such arguments in the first place. If interest rates are purely the result of expectations (term premium equal to zero), the purchases themselves have no direct effect on yields, only as the result of signalling what the Fed will do (the first channel). However, the Fed could achieve the exact same effect by just saying that the economy stinks, and they will not raise rates for a really long time.

In order to find a justification for the decision to purchase bonds (instead of just talking), he needs to argue that the change in supply affects bond yields. The problem with that argument is that the front end of the yield curve does a pretty good job of tracking expectations, regardless of the state of supply and demand in the bond market. (One can look at how market participants use front end pricing as an estimate of what the central bank will do in the next few rate setting meetings across all the major markets as an empirical validation of that assertion.) 

Therefore, we need some reason to believe that the short maturity bonds are somehow different from long maturity bonds, i.e., there is some form of segmentation. This way, one can hope that supply and demand affects long-term bond risk premia more than short-term premia (which would sound plausible to most bond market analysts). What matters is that the term structure of the term premia is changing due to the supply effect. One can come up with different verbal explanations for why term premia move, they will end up being indistinguishable in practice. That said, we need to ignore the possibility that other factor affect term premia, such as returns volatility (which would be suggested by portfolio theory).

The obvious issue with this argument is that Fed purchases are not the only factor affecting supply and demand in the bond market. Why are those other factors being ignored?