I don't think there's anything to understand, aside from Delvin being a petty jerk. It seemed like his big problem was that he believed Nancy messed up the buttonhole foot, which I doubt. I have a Brother sewing machine and it has a buttonhole attachment that is relatively idiot-proof. You pop the button you're using into the guide in the foot, snap the foot onto the machine, pick one of the gazillion types of buttonhole stitches, and floor the pedal. The machine stops when the buttonhole is completely sewn. You don't even have to change the threading, unless you need a different color or something.
I re-watched the buttonhole drama piece and the buttonhole foot is the same as on my machine. So the way I read that was Delvin was in a snit with Nancy and was looking for anything, no matter how minor, to put her in what he thought was her place. It seemed like Christian wasn't having it and stepped in to let Delvin know that he saw right through that BS.
I honestly don't know who's the instigator - Delvin, Brittany, or Victoria. It's entirely possible they all feed off of each other. Victoria can shut all the way up on Nancy repeating a pair of pants. She is the absolute queen of variations on a theme.
Both Brittany and Delvin seem like mean girls who never mentally got past high school. They both strike me as insecure and needing to put someone else down to reassure themselves. Brittany specifically kept her voice low, like she didn't want people to hear her talking shit. And Delvin had no problem going in on Nancy when she wasn't in the room, but the second he had the opportunity to say it to her directly? He got quiet in a hurry. I'd bet he did the smirk as a way of saving face rather than looking cowardly.
The judges seem super convinced of Victoria's ability to fit. I...don't know about that. I think she can fit a tall, thin, flat traditional sample size model. Which is about as easy as it gets, in terms of fitting. And it's possible that she's picked fabrics that have a bit of stretch or are very forgiving (specifically her blue dollar store David Bowie outfit). Her sheer fabric, which had no stretch looked like a hot mess and this satin dress was everything people hate about satin - it looked cheap, ill-fitting, and kind of tacky.
On a positive note, Marquis' model was so freaking good. I felt bad for him that everyone was treating him like a barely-sentient piece of meat. It wouldn't be okay if they had done it to one of the female models and it wasn't any better just because the model was male. The model had crisp posture and was really selling the hell out of the regal vibes Marquis' garment was giving. I think Marquis' look deserved to win from a design perspective, but the model giving the right energy made it stand out that much more.