If I remember right, if he establishes his headquarters store in that town he isn’t obligated to do spy work for the country, (he would have to do that if his headquarters were anywhere else). Doing this allows his store to spread to any/all countries since they don’t seen his chain as an enemy spy. Rotsgard merchants do spy, but the country being pretty much centered around the academy town, it’s in their best interest to stay neutral, to be able to get students from everywhere and get influence that way, avoiding fighting altogether. Him attending the school is trying to get access to the library, look for clues on how to defeat the goddess, (and learn what common sense is, maybe?). The school thing has purpose but it seemed like it was on a similar level to a whim?