As for the distinction between Fed and Treasury, let me understand: The Fed issues the legal tender and decides how much to create. The Treasury uses Fed notes to pay its debts. How are these things separate?
If I decide to buy US debt, I am relying on the US to generate the revenue to pay the interest, and I am relying on the Fed to uphold stable monetary policy so that the interest is repaid in units with reasonable purchasing power. An investment in US debt requires both Fed and Treasury to support the asset.
The Fed has willingly taken on bad assets, and it seems to me, that the impetus for investors to continue buying the debt is associated with the strength of the economy, Fed balance sheet aside.
It's like the opposite of a strong currency policy. If I back with gold, it's somewhat secure. If I back with trash, what's the upside of that? There should in fact be a penalty... but there isn't in this case due to the US economic backdrop. Which brings me full circle. The Fed balance sheet such as it is, is sort of underwritten by US economic power--created by the US public.
Sorry, this is a simplistic description; Obviously I don't follow the Fed balance sheet to the degree you do, but I'm (over)simplifying to avoid confusion. I often feel like many of the details of central banking are obfuscated to the point of mystifying them; I'm aiming for clarity I guess.