CurriMintz - Same here. The art is great, but the story has flat-lined. I put it on hold and will wait a month of so before going back to it. Grace has been abandoned and wasn't Lynn's heart-health declining really fast now?
The issue of the life span disparity is probably unavoidable. Kanna will live on after Saikawa's death. However I think it's plausible that Kanna's appearance would change along with Saikawa's, as she mentally matured in the human world.