Yea, you're putting a bit too much significance on Harle and expolating her power. She was just a component for the Dragon God's completion made to masquerade as a human or demi-human. She's associated with the Dark Moon probably because it was already there, otherwise she'd get a star. Not to mention that the connections are symbolic only. They have no spiritual ties to the astral bodies.
As for Harle's emotion, that's not coming from the planet. It's coming from herself. The irony is that she's the most human of any of the Dragon Gods, based on Schala's DNA and everything.
Don't make her seem so useless.
Anyway, shes not necessarily a component, but she does have her mission. She has to get the Flame from Chronopolis, and then her brothers and sisters would be free to merge. She did not necessarily have to merge with them. One of the endings where the Dragon God merges has Harle not merged with them at Marbule with the Sage and Serge in Lynx's body.
I know it's symbolic, but we already have so many quotes on her and the moon, and her power is that of the Dark Moon. If it didn't exist, would she be able to use her skills? She obviously has manipulation over the Dark Moon and it's light (hence she has Moon Beams and Lunateric). If Belthasar wanted to make sence, he should have said something more along the lines of "One star split into 6, combined they made the 7th" as an example. The problem is that Belthasar doesn't give the one Dragon God an astral body of it's own, unless the Dark Moon is that, and we know that Harle is almost like a mini combined Dragon God in her own right.
What I was meaning by her emotion is that if she, like Razzly can feel the planets sorrow, then Harle would be the true Dragon God - the uncorrupt one as everyone seems to think about the Dragon God being tainted by Lavos. Schala's part would have played when Harle would have been crying about leaving Serge, because she has to stick to her kin (reminds you of Schala eh and how she couldn't disobey her mother).
The Dragon God is connected to the planet, not moons or astral bodies.
But everything in the planet is a part of the planets elements, and the Dragons are just that. Had there been no Ultranova's in the universe, then how would a dragon based on those powers be able to exist when the planet itself is not a galaxy and is not able to reproduce that. The planet may have gravity, but it surely isn't a black hole.
Remember, the dragons are the biological incarnates of the elements. Heck, the fire dragon was standing in Lava and wasn't getting hurt.
Also, I bet there are other power spots in the CT universe. Starky can surely use magic, yet he's not from the planet originally, and neither would I think that he just walked into a town and bought himself some elements.