The OC job will probably go to someone whose name hasn't been leaked to the media and hasn't been discussed up to this point.
It does sound like after Fulmer's first two choices, Spence and K Brown were out he didn't have a third choice in mind and is starting fresh.
From what I understand about Marrone, he probably makes 500k as an NFL OC. It's difficult to imagine he would return as college OC to make around 300k. Doesn't seem like a good move for him.
You have to at least consider members of your staff for a position like this, or they end up feeling slighted and leave. I think that is what Fulmer is doing with Adkins.