I don't know there's a lot to unpack here; puzzle box may be a bit much.
the film starts in shocking fashion, but shocking and unpleasant are very different terms, and referring to the opening scene as unpleasant is pretty cruel body-shaming.
I agree. In fact, for me it set up the only coherent thread I could find re: gender roles. Adams hosts this exhibition (wish she textually "doesn't get") that celebrates and emphasizes the bodies of women society may otherwise disregard. Adams is a character in complete contrast to these women, neither joyous nor free. She's a self declared realist, and acts entirely in accordance with social views on beauty and ambition. It is for these reasons (kind of?) that she leaves Gylenhaal.
Wasn't his weakness in the novel, and failure to save his wife/daughter, a comment on his perceived weakness in real life and inability to keep her and save his unborn child?
I think this is right. A bit extreme of a response to a break up, but the logic tracks.
I think it is as simple as he stood her up as revenge for her leaving him and aborting his child?
This makes the most sense for me, unfulfilling as it may be. I think the idea is that the book proved Gylenhaal was strong for sticking to his creative aspirations and creating something Adams actually engaged with (making her abandonment for some one more practically minded all the more meaningless, at the beginning of that relationship's decline). I don't know there's much more there :/
A potentially controversial question...
I'm doing my best to empathize with Gylenhaal after he learned his s/o aborted his child, but I don't know the film makes a good enough case to also describe Adam's choices as "unforgivable" (which, it does). Let's say, I reeeeeally loved a lady, and she was pregnant with my kid, and she aborted that kid without my input or whatever. I don't see that as an act, in itself, too far to return from. There are potentially religious implications that don't particularly effect me, but like i said, the movie doesn't really try to work any of those arguments in. The dishonesty is one thing, but it isn't like infidelity where the act signifies "i want something different". If my s/o didn't want my child, I wouldn't insist they still have it, and I wouldn't take it as code for "our relationship has no potential future" as Gylenhaal seems to. Like, they can get pregnant again in the future if that's what they want; if anything its rational to avoid children until you're confident in your relationship. which Adams previously made resoundingly clear, she wasn't. Maybe i'm too young or something, but i really don't get the big deal here.