2&3. Democrats "running on" some policy does not at all mean it will happen. In the current political climate, it's quite likely their holding a position means it won't happen.

4. Large firms have already been using record low interest rates to finance stock buybacks. This has been going on for years. The last three or four years of stock price growth has been fueled by financed buybacks, with every year breaking the previous year's record. At some point will need to stop.

As an aside, these buybacks are likely going to be the cause of our next stock market calamity.

5. This is sort of legit fear, but 3 trillion dollar is not really enough to make a market-wide impact and selling their bonds would just push interest rates lower, serving to help the US achieve their current monetary policy. They could use this money to selectively certain industries though.