The guild generally only does business with regular lay and mednobles, and lockpicking isn't really a regular spell taught at the Academy. That's why it's so easy for Eckart and Justus, who are archnobles who do the dirty work of Ferdinand.
It's less lockpicking and more that 2 archnobles showing up weren't expected in this system at all. Remember, most nobles
in the capital don't even deal with commoners. Normally, the lock would only be used by a specific part of commoners: wealthy owners of business in the capital of the duchy.
Also LN extras says they simply register their mana on the lock (so it's like creating a new login) rather than actually lockpicking it. I was under the impression its magical locks were on the door leading to the top floors, instead of the street entrance. Oh well.
So.. the woman at the clothing store is the same that is friendly towards Benno when he is taking Myne to buy some new clothes, isn't she? I dont remember part 2 of the manga atm, cant remember if we were ever shown how she looked like (LN doesn't describe her looks)