Also, the more I think about it, the more certain I am that Willem was the one behind the thugs. First, the motive: he likes the MC, and wants to look good in front of her and get her to drop the pretense of being the prince's fiancee quickly. That is to say, the attack was fake.
Now, the evidence. First, as she says, the timing, and that she was targeted at all, is suspicious. The thugs targeted her specifically for being the prince's fiancee, but that fact is not public yet. Plenty of people might be jealous of her, but very few would know of her position. And was there any need for the person who hired them to go into details? They would just need to give them a description so they can find her, not an explanation for why they have to do it. If anything, telling them they're targetting someone of high status just makes it more expensive, and if they're disposable pawns, the less they know the better. So the person who hired them wanted them to specifically say that they were doing it because she was the fiancee, so that she'd be shaken up and give up the role.
Also, finding her in a crowd is strange, and even more so setting up an ambush. As she said, this was the first time she went on a date out to town, and she disguised herself to not stand out. She also doesn't show her face regularly, so people wouldn't know who she is. It doesn't look like there are photographs in this world yet, and it's hard to imagine that someone can give out a precise enough description for a bunch of thugs to identify someone they never saw. I suppose it might be possible to provide small paintings, but that takes time and it would be difficult for an enemy to easily find a picture of her to copy just to show it to thugs. Plus it would leave a trail of the artist hired by someone unrelated to the duchy painting a portrait of someone else's daughter. And possibly a small sized one that can be carried around to show to thugs. That would probably stand out to the artist.
Even if they were somehow able to identify her, knowing she would be there that day is impossible. The only two people who knew about the date are Willem and her. Not even the rest of the team knew. Why would the person who hired them tell them to lie in ambush, when she never goes out in the first place? And yet, they were able to not only find her, but predict her path enough to set up an ambush in an alley. And here is where it gets extra suspicious. Glasses was the one leading her and the first to spot their pursuer, and yet he led the two of them right into the ambush instead of looking for somewhere with more people. And even when they escaped the first ambush, instead of going to the main street as she told him, he led them further away from people, into a dead-end. And yet he was perfectly confident the whole time.
Then his performance in fighting them off. It could be that he really is that good with a stick, but it's still suspicious that he defeated the lot of them without anyone being seriously hurt. To add to the suspiciousness, after they ran and he took her home, he allegedly returned to the scene of the fight, and managed to interrogate them further. Wouldn't their friends have picked them up and hidden them? Were they really uncouncious the whole time? We only have his word for it that he really returned. And besides, if he returned and interrogated them, why not hand them over to the guards? Is just getting a bit roughed up really enough punishment?
The main counterargument to this theory is that we can supposedly read his thoughts, and he does appear surprised by their appearance. But, authors are known to cheat and that might have just been misdirection. He is the perfect suspect except for that slight detail.