Fed monetization of debt is a net add to the system. They reinvest any principle payments.
Once the achievement of improved employment & housing has been accomplished, inflation would have picked up, be it 5-10-20yrs. They can then extract money back out of the system by becoming net seller, neutralising in theory inflation at that future time.
Derivs you mention is old hat, many a novice on HC doesn't look beyond the sensationalist numbers.