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.
Tohru even says that the world Kobayashi lives in is low in mana, so it's likely that the dragon's life span comes from that, and that without it, they will age as other humans.