While I don't know what it's like to be offered an amazing magical ability that has the potential to be used in an inappropriate manner, I do know what it's like to be walking in the general direction of that one cookie place at the mall, resolutely deciding not to buy a cookie, and then finding myself eating a cookie a minute later.
What sort of things could Nanase hypothetically do with her magic and a watch that can make her look like someone else? Heck, just look back at the second comic of this storyline and read from there to get an idea. Then reach this comic and this commentary, go back to the second comic again, and read back to here again in an endless loop.
Or maybe don't get stuck in an endless comic reading loop. Whatevs.