I agree that this is a seinen title. The traditional demarcation has always been where it was published, and if it's in a seinen publication, it is seinen by definition. There are always going to be titles with some cross-over aspects and you could argue forever based on your subjective evaluation of how much of what aspects in a particular title makes it a seinen or a josei, but that's why there is that hard and fast rule: if it's in seinen, it's seinen. Same for josei, shoujo, and shounen. I finished this about a year or so ago, and Kotarou and his circumstances come to forefront quite a bit more and he becomes at least a co-protagonist. Toward the end, he drives the narrative more definitively.
I really love this manga and am glad it's being introduced to a wider audience.