Not sure he would have completely moved on before Molly actually. It seems to me that he/they may have been in a state of limbo for a while - still living in the same house trying in vain to rub along, until right up to his fourth tour, when Rebecca was sorting everything out, and the decree absolute arrived during the tour.
When Molly finds the document in a box along with his son's photos tucked away - I think the separation (from his son) was so raw and painful, he couldn't bear to look at them so had put the box out of the way. Which is why the pictures weren't displayed around his quarters.
It seems quite raw to me - the hospital interaction between CJ and Rebecca appears to hold a lot of sadness and hurt - not like something that had been over for a long time where they've had time to make adjustments. "Oh don't leave on my account" she says to Molly bitterly. Perhaps even though neither had been happy for a long time - there hadn't been a chance for closure until that point.
Also, it would have been easier for CJ to not think about how things were at home while on tour, and there is also a resonating theme with Molly too "On tour everything seemed possible. Being back home is like waking up". For both of them being on tour provides a form of sanctuary from issues at home.
I think if there is a second series there would be an opening for Rebecca/Sam to reappear in some way...we've seen bits of Molly's home life that hasn't detracted from story. As long as it doesn't become a domestic drama!