I think this is a clearly a case of ABC misleading fans with its promos.
Undoubtedly, it will be awkward for Castle having Beckett and Meredith to contend with (and probably Martha winding him up). But I seem to recall a comment (possibly from Marlowe) in which he said that any tensions or problems between Beckett and Meredith are resolved quickly - and then they move on to have some fun with Castle. Who knows - it could be a case of the girls winding Castle up.
There's also the "You're not the man I thought I knew". I think everybody is missing a big issue here; for over 4 seasons Beckett has believed that Castle is a womaniser. Two marriages/two divorces do not harbour well for the future (Beckett and Lanie had this chat in 4x20). Beckett still thinks that Castle is the culpable party in the breakdown of two marriages and he's done nothing to disavow her of that notion. What's to say that in this episode - at long last - she finds out that Meredith left Castle holding the baby? What's to say that Beckett realises that the man she thought she knew - the womaniser, the one with two failed marriages, etc. - isn't real? That Castle isn't the man she thought he was but is, in fact, closer to her ideal of a partner, husband and father. It's a wake up call to Beckett - she's been too wrapped up perhaps in her own past to take a look and ask the really pertinent questions. Castle has stood by her and discovered things about her but she has barely scratched the surface of Castle's life.
Castle isn't the man she thought she knew. He's the man that we all knew he was but Beckett didn't see it. This might just be the chance for her to commit more fully to the relationship.
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