I'm not sure where the certain conviction, that C and B are in conflict or on the verge of divorce, comes from. I got the impression that the first 2 eps will be of the same story told from 2 points of view: Castle's first, then Beckett's. I'm convinced they will be "separated", sure, but only by circumstances, not conflict between them. I envision this pattern:
801:
- Castle in his PI digs, on the phone to Beckett. The conversation serves to tell us what's happened. He's taking on clients, she's been promoted. We don't see her, but his lines tell us what we need to know. They make a date for a rendezvous: lunch? dinner?
- Other stuff happens (Alexis? Martha?) that further brings us up to speed.
- Something inaugurates the Case of the Week: A Bad Guy does something... a client comes in...something.
- Castle takes action, or misfortune befalls him. He's hurt! He's a prisoner! He's deceived!
- Cliffhanger!
802:
- Beckett's turn. We see "Captain Beckett" for the first time, settling in to her new office. Precinct action introduces us to the changes in command structure & personnel.
- Beckett realizes something's happened to Castle!
- Beckett mobilizes resources to resolve the case to find/help/rescue him.
- Blissful reunion!
Maybe there's a new Bad Guy that will haunt the rest of the season, or it's Bracken back to poison Beckett's triumph, or some new engaging twisty thing.
Maybe there IS a tragic angsty undertone: a secret Castle has to keep, a seeming-betrayal, a spurious doubt. All grist for the mill.