Man, what a cute, beautiful and lovely journey. Sad it's ending but it was all sweet.
I also agree with the fact that Dahaaka could have had a better place in the story, plus the miss opportunity of her reunion with Dou.
I'm also not entirely sure why Dou couldn't continue living with them without the act of the Demon Lord. Sure she became weaker over time, but she was with them for a "long" time before that, so was it not possible to have her recharge her batteries by going to the tree like once or twice a month or something? Dou also planted a tree near or in the castle, so it could have serve as her resting place too. Unless I missed something about how Lunafian/Dou's condition is working? I guess the causality stuff is probably the reason as the old Lunafian said they could only come at important time, but I'm still not fully convince about such explanation...
Anyway, good read, totally didn't cry to all that sweetness!