@15Redstones Poor, yes. But I'm pretty sure being "rude" had nothing to do with it. Per the text, Bezewanst had already made up his mind before Myne's parents spoke a single word.
If Myne had walked in there with somebody like Gustav as her father, the negotiations likely would have gone smoothly. But that wasn't really an option. Even Benno couldn't really get involved. Well, he could have made Myne let him adopt her and come himself, but Benno wasn't that interested in risking his own skin on a hail mary. Also, Myne would have refused that option. All this is more complicated because Myne only had less than two days to prepare, and she was laid up in bed on the day prior. Also, her parents were at work. In the end, there really wasn't any way to train her parents for the negotiations, and through circumstance and her own choices Myne couldn't come with substitute parents. And while it would have been hilarious to watch Bezewanst slowly realize that the two well-dressed adults in front of him were basically country hicks, putting them in clothes above their status wouldn't have helped for more than 30 seconds after opening their mouths. Thats why I don't think that Myne's parents are at fault, there was no way they could perform what they hoped to do with Bezewanst, who changed his mind the second he laid eyes on them. Benno's carefully laid plan was already useless before they could even speak.