We see Ennis in this scene embracing his daughter Jen, and also in other scenes embracing Alma, and Jack, and even Cassie when he dances with her. But where do we ever see Ennis being embraced? The abandoned boy surely needs to be embraced, to feel the safety of arms around him. The closest thing I can think of is when Mrs. Twist's hand touches his shoulder. And the hand of his father, when he was nine years old, forcing him along to witness a horrific deed...
Well, the most significant time that I can recall Ennis being embraced is TS2 when Jack pulls Ennis on top of him and sort of cradles and rocks him. Actually, I think almost that whole tent scene from the moment that Jack caresses Ennis's forearm to the kissing at the end is Jack "embracing" Ennis since he's the one in the lead here or guiding Ennis along.
I think that Jack's Mom at the end is a great example of another kind of "embrace" directed towards Ennis. To me it's one of the most poignant moments reminding us of the fact that Ennis is an orphan (the other scene that always reminds me that Ennis is an orphan is the dozy embrace when he mentions his Mother). Mrs. Twist definitely seems like she could have served as a surrogate mother figure (or doting mother-in-law figure) if Ennis had pursued a more committed relationship with Jack.
And, as you suggest about the topic of the nursery scene, I think this also may be about Ennis as a father in light of the fact that he's an orphan and had a horrible role model for a father. His tenderness towards his daughters and his willingness to help Alma with the children, I think go a long way to show how different Ennis is from his own father (or at least what we know of his father).
The image you include in the first post is really, really interesting to analyze. In that shot we see Alma separated from Ennis through multiple layers of glass and presumably she'd be able to see him through those multiple windows (even if it would be a fuzzy image). In this image she'd be seeing Ennis as she would want to see him, as a devoted husband and father. Later, she again sees Ennis through glass (the glass of the screen door during the reunion kiss). When she sees him kissing Jack her view is much clearer with only on layer of intervening glass and maybe she's getting her first clear view of Ennis as he really is. The great issue of
Film Quarterly devoted to BBM has a long article in it titled "On the Universality of Brokeback Mountain" by D.A. Miller that's divided into many sections. One section of the article is called "The Homosexual Observed: Erotic Play Under Glass" and it's about heterosexual characters (Aguirre through his binoculars and Alma through the glass door) observing intimate moments between the two gay characters through the mediation of a glass device. This nursery scene seems to be another interesting, and very complicated, moment of Alma perhaps observing Ennis (a gay character in this case not with his partner) through multiple layers of mediation.