It's a complicated situation for sure, and I suppose we, as readers, have a different perspective on both Mikko's and Arvo's mindsets about each other that they don't have, but if I was just a friend even, I would feel very uncomfortable with a random guy coming in and flirting with my close, childhood friend. Especially if I thought he had bad intentions for them.
But then again, you're right about your prior points against it.