spoiler reply
For me the issue isn't that there isn't a good ending but rather that the aggressiveness of it was pretty out there. You're definitely right about the red flags though, especially when you consider the first bad end you can get in Mikko's route.
I don't completely agree that Arvo is his Torulf though. I guess from Mikko's perspective, sure. The shit that happened was like in junior high I think it was mentioned. While shit can carry with us I highly doubt that's the kind of trauma that would create such a sadistic method of dealing with shit. What's more likely is that something way worse happened afterward (the mentioned boyfriend maybe?). Then Arvo comes along and he tried desperately to cling to something positive but ultimately uses Arvo as an easy punching bag instead (Arvo is kind of a push-over).
It should also be noted that Arvo had tried to avoid Mikko for the most part. Sure it was ultimately his choice when the key goes missing but Mikko was the one who tried to reconnect first. I don't think it's fair to blame Arvo for relenting to reconnect, especially when he's clearly not good at seeing red flags. I could see why you would feel awkward about Mikko's route being a route but in Arvo's perspective at the choice, this is him reconnecting with a friend not choosing a romantic partner.
That said though. We don't know the details on other things and his behavior throughout are signs of a very unhealthy person. It will take a lot to turn the route around and have it not be toxic and if I were in place of Arvo I wouldn't even try after the blow up (And get the police involved in terms of the bad ending). I'm gonna say it, Mikko is a bad dog! Bad dog!