A big part of the problem is a lack of communication. If she is a Fashion Nova aficionado, I would not have left the leeway for them to return/change it. You opened the door to all of this. Her wearing something else, therefore, was seemingly appropriate to her. There was room to do that. If it were THAT important and I know that she's fragile, I would have confirmed the outfits beforehand, so there were no suprises, even if I had to go shopping WITH her. Just because you paid for/planned everything doesn't mean she has to go along and since you opened the door for flexibility, this happened. Once already at the shoot, I would have asked for some pics with the planned outfit just to have "options" and made sure I got my pics that I paid for.
Also, you seem to kind of not like DIL and it comes through every time you talk about her. I think that is playing a role, too. There's underlying or unsolved issues also at play here. I find it interesting that so much of the posts talked about her belly, body, tacky/"creative" body con wear, her fragility, petty ways to get back at her by using the big belly pics anyway, to cropping HER/them out, family comments about her dress and like one line about your son's cut off shorts? Ya'll already don't like this girl and it's obvious. She's tolerated
. Try to fix it. I agree that in a case like this, though, common sense should have prevailed (wear the damn free outfit!), but I'm learning that common sense isn't all too common.
Lastly, your husband is being extra petty and kind of controlling, but he's hurt and I get it. I just hope his reaction is temporary and that he won't make any long-term decisions based on this incident.