The Bills declined the Skins' request to interview him for DC so they clearly want him to stay in at least the DC capacity.
If I'm the Bills FO, with Schwartz already in the fold I hold out for a Quinn or Bevell, whichever agrees to keep Schwartz as a full charge DC. If either of them decline, I have a fallback in Schwartz as HC, maybe with Pepper as DC and a Trestman as OC.
Not sure how Schwartz could decline either scenario.