Hasn't the narrative of Pete not wanting the job been debunked? I believe he wanted the job but understood that the team was going to interview people outside the organization and not just promote him.
Yes, I remember reading something on here that quelled that rumor. And no, I don't have a link.
A new OC is most probable. I say this because new coaches would typically get 3 years to turn a franchise around. Of course, that doesn't necessarily seem to be the norm today.
As far as where the new OC will come from, I have no idea. I know it won't be a lateral move by Johnson, Smith or Waldron because those guys will be head coaches next season.