Hallmark-inspired J/B AU crack because... well, it's been a while since the last AU crack and tbh I was kinda starting to miss it! And also, I recently ended up rewatching this silly comedy from 2012 called Undercover Bridesmaid.
So, the movie is about this tall, blonde bodyguard from a private security agency. Some rich Texan businessman hires her because his daughter, who's about to get married, has been getting death threats from an anonymous harasser and will most likely make herself an easy target during the ceremony. The daughter is a ditzy airhead chasing dreams of romance and a perfect wedding and won't take the threats as anything but a joke in poor taste, so calling off the whole thing for now is not an option, and neither is having a bunch of big menacing guys in suits shadow her every movement on her big day... so the rich Texan has no choice but to hire the one female bodyguard the firm can offer and task her to befriend his daughter so she'll want her as one of her bridesmaids, allowing her to do her job without being noticed.
Which actually does happen, to the point the Texan's daughter actually starts to consider the bodyguard a dear friend, and even the bodyguard, a very strict and closed-off type, starts warming up to her. She also starts to warm up to one of the groom's friends, though, which is a problem because she can tell he's hiding a secret and that makes him a possible suspect...
Ofc, there's lots of shenanigans, including the obvious "woman who's not very girly at all and is very serious and tough gets stuck with a bunch of other women who love all things girly, mushy, and romantic" humor and "everyone is hiding a secret but no, nothing to do with the death threats" subplots. And ofc, the bodyguard has a tragic backstory where she was left at the altar by a shitty boyfriend so now she hates all weddings...
Eventually, things come to a head on the wedding day, when the groom's friend decides to confess his feelings to the bodyguard while also coming clean about his secret (he isn't some rich law graduate like he's been telling everyone, he's actually still doing a bunch of odd jobs to hopefully earn enough to start studying law some day soon)... but to hear him out, the bodyguard forgets her duty for a moment and lets him draw her away from the bride and her friends, and that's when someone tries to shoot the bride.
She runs back in time to prevent anyone from actually getting hurt, but the rich Texan is beside himself with rage and fear and starts berating her in front of everyone for walking off with some guy instead of doing her job, blowing her cover, firing her and telling her she'll replace her with one of her other colleagues (and to hell with "perfect wedding" appearances), and demanding to know if she's made any progress in finding out who's trying to harm his daughter all at the same time. The bride feels hurt because she thinks her new friend was only pretending to like her as part of her job, the groom's friend is hurt because he told her a secret he felt ashamed about when she was hiding from him that she was on the lookout for a whole assassination attempt, and... pressured by the rich Texan and unable to explain herself in the ensuing chaos, the bodyguard lets slip that she briefly suspected the groom himself but apparently his secret was just that he was slinking away with some other woman. Only to eventually resist the temptation out his genuine love for the bride, sure, but again... angry shouty fathers and people going "wait, you're a bodyguard!?" everywhere and words not coming out like they should.
Dejected and feeling like she's failed on all fronts, the bodyguard still sends the bride a video showing the groom not actually doing anything with the other woman. After receiving it, the bride forgives him and doesn't call the wedding off, just postpones it, but when she tries to call the bodyguard (presumably to say something like "I'm sorry I reacted so badly, I hope we actually are friends, will you please come to the wedding anyway?") she doesn't even pick up.
So the groom's friend figures out what's going on and goes to visit her at her firm, where's training in some kind of gym room, and there's an actually quite sweet scene where he puts on boxing (?) gloves and lets her punch them again and again to finish exercising (and probably also to vent, she needs it) while he tells that he should have understood right away that she was only hiding stuff from him out of necessity, that nothing's changed between them, and that she should let herself free to feel and to care for and be cared for by other people instead of trying to keep everyone at arm's length like she's been doing ever since the "left at the altar" thing.
He finally convinces her to show up at the wedding, not as a bodyguard but as a real bridesmaid and friend of the bride this time... and, obviously, she ends up figuring out exactly who the real culprit is (it's another bridesmaid, whose family business had been cheated out of some lucrative deal by the rich Texan) and stopping another murder attempt in the nick of time.
As you may have already figured out (XD) I'm thinking of Brienne as the bodyguard and Jaime as the groom's friend. Her initial uneasiness around the other bridesmaids would be easy to translate into her comparing herself to a bunch of conventionally pretty girls in very feminine dresses, and her dislike for weddings might come from The Bet either going WAY too far and somehow ending in a fake proposal or from her learning about it at a wedding where she naively agreed to come as some jerk's plus one. And his secret could be that he's been cut off by Tywin and now he's struggling to make his way in the world all on his own unlike all the other rich people in the wedding party. The boxing scene would, ofc, still happen. ;)
I'm less sure about the bride and her father, but I was thinking... Sansa and Ned, maybe? She'd be a lot less of a "spoiled sweet" airhead than her movie counterpart, but she could be marrying Sandor after getting out of a bad relationship with Joffrey and too anxious to put the past behind her and "seal the deal" on a brand new start to think clearly, making her more romantic and dreamy side resurface. While Ned would just need to be less of a rich ass (really, it would only require a bit of a different attitude and a different motivation for the villain), but he could still lose his cool with Brienne right after witnessing a murder attempt on his daughter.
Sandor as the groom, too, would need a different subplot than "he has a bit of a wandering eye but really he's loyal"... but the "other woman" could turn out to be someone he met in a support group for victims of familial abuse and only there to offer emotional support in case all the wedding preparations and the tension risked making him freak out in front of Sansa's family.
OKAY OKAY OKAY SO I HADN'T MANAGED TO CHECK TUMBLR UNTIL TODAY AND I WENT LIKE WOW HAPPY BIRTHDAY TO ME THIS IS THE PERFECT PLOT TO FIND IN MY INBOX SKLFJGDLJKDJLKSGLJDGKLJKGDGLJDKGJDKLJKDGLKGD OH MY GOD THIS IS AMAZING AND I WANNA WATCH THIS MOVIE NOW but in all seriousness your casting sounds good to me and tbf if jaime had to be the groom's friend then idk if anyone but sandor cut it and sansa/ned would work perfectly (ALSO I WANT THE BOXING SCENE NOW I'm gonna find a download for this thing pronto) I mean you can just make ned less of an asshole and the fake bridesmaid can be someone who wants to fuck ned's ethical business over I mean ngl could be roose who is in the board for ned's company and wants to take over so he thinks having sansa murdered and framing sandor would make him step down (???) could be an idea or could be cersei who hates sansa bc she broke up with joffrey and it had a bad public repercussion and is prob not even invited but brienne could just do her investigations and find out (DUNNO IF ANY OF THIS SOUNDS GOOD TO YOU LMK)
and ngl for sandor I like what you said but it also might be that in this version after Shit Happened™ his sister ended up adopted while he ended up in the system and she looked him up/found his info and wanted to reconnect so he's meeting her but without telling anyone bc he's not sure/he has trauma/he doesn't want her to end up in a mediatic mess bc he's not exactly marrying a no one and she's actually offering emotional support and when brienne finds out she's like okay sure you're good you say when you feel like it (??) but yeah I love everything about this I'm off to find a torrent THANK YOU SO MUCH FOR THIS GENIUS