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.
That's of course assuming that a dragon's longevity is due to an inherent factor rather than magic or something external. What I mean by that it is possible that time flows at a different rate on the plane where dragons originate from and that coming to Earth may give them a normal lifespan like demons in Disgaea. If this is true, then the reverse might also be true as well which would allow both of them to stay together forever.