These mechanisms you mention are all mechanisms that have historically been used, but there is a widespread consensus that it's not the right thing to do. Best practice is that a central bank should be independent of government and their mandate should not be to meddle with the valuation of a country's debt but to keep the value of the currency stable. For example, for the ECB it would be completely impossible to make monetary policy to suit the fiscal needs of all Euro-area countries since they are in wildly differing situations as far as their debt-levels and fiscal policies are concerned. Fortunately that's not part of the ECB's job description, though. Their job is: Keep inflation at a target that's close to but not exceeding 2% over the medium term. In theory the Fed should be just as independent from the U.S. government as the ECB is from any European government.
Of course if you believe anything you just said about the ECB you are really delusional. They are very much dependent on a few European governments (primarily France and Germany), this became readily apparent during the financial crisis of 2008. The Bundesbank president at the time had served in the Kanzleramt before.
The national central banks don't have all that much to say within the ECB system and are basically a historical remnant of a system that has been superseded and made obsolete (like the queen of England).
Personally I don't have a problem with somebody serving in government administration moving on to a role that's independent of government. Not everybody in administration is a political entity, and it is very possible for somebody who had been a political entity to leave politics behind. The same people who like to spin everything into conspiracy were also keen to point out that Mario Draghi had previously been with Goldman Sachs. ...so if it is somebody from public administration, there's a conspiracy theory. If it is somebody from business, there's a conspiracy theory, too. In that case where are these people supposed to come from? In my opinion it's a good thing that people need a proven track record of some kind or another to be eligible for offices like that, you can't just pick them up on the street just because, for political reasons, you require that they should have no past of any kind.
And, finally: European sovereigns weren't particularly in trouble in 2008. It was in 2010 that the crisis had made its way into the accounts of sovereigns, most notably the Greek debt crisis which later expanded into a debt crisis for Portugal, Ireland, Greece, and Spain. During that time, this mechanism was, indeed, a much discussed topic, but precisely in the sense of what I previously wrote: It was a historical precedent for how a sovereign debt crisis plays out when you can't just inflate your way out of it.
