Well, let me write down what i think is the meaning of this anime.
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warning, much reading ahead
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I’ll start with a short, somewhat chronological, summary of what happened:
The planet Mars had been succesfully colonized bij humans, though be it only in domes, outside life is still being worked on (Cross working with his plants and finally taking the risk of -succesfully- going outside). Children are being born inside the dome and it’s a happy and good place to live.
Then for some reason the moon comes close and disaster looms. Which moon? I personnaly think the moon from earth is a possibility since the ship that carried Layla came originally from earth and then from that moon to Mars. The original dozen decided that they cannot harbor those extra people for some reason and decide to shoot them down. It is from then that there are no more children being born. (The timespan between that ship going down and the endfight is 13 years, according to Vespa:”13 years ago, Volk began to walk the path of demons”)
But with the barbaroi who had nothing to do with taking down that ship one child (Nei) is born. Mars is making a new start, but not with the original people.
This child is protected by another child, that ironically should have been killed with that ship (Layla).
The original dozen must pay for their sins, especially Volk, and be removed to make a new start for humans at Mars, which is symbolized by Nei and her ‘maturity’.
As for the characters and that red moon:
As for Nei, I think she stand for the life that the planet Mars can give. This is mainly because in the beginning she isn’t alive at all, she is a doll. When we find out that she s a girl she is just a 10 year old girl. As the red moon comes closer she get’s ill and in the hospital she changes from blond to pink haired. During that last fight her transformation is completed when both her eyes become pink (why pink? It’s the color closest to the red of Mars) I’ve seen the Anime-Keep subbed version and Vesta says there that “Nei has been sexually immature. But now she is....” Meaning there can and will be life again, not just from Nei, since she is a symbol for life at Mars in general.
But besides Nei, Volk also stands symbol for life at Mars. He was the leader of the original dozen and only after they stained their hands no more children were born. When his body is damaged the surface of mars is ripped open as well, bringing fort water (which is needed for and symbolizes life). But besides that, both his and Vespa’s bodies needed more and more care as the moon came closer. Cross (who was opposed to shooting down that ship) didn’t need that at all...
For the both of them (Nei and Volk) counts that they stand symbol for life at mars, though be it for different people. They even seem to share pain (Nei felt pain as Layla hurt Volk) and i think she could even have been killed when Layla had pulled through and had killed Volk (I think Layla realised that and that’s why she pulled her hand back, letting Volk live...). And they both react to Mars: as Volk is bleeding and mars is filled with water and starting to live and all, then Nei feels better as well. Volk realises it too when he looks at Nei when he has lost the fight and looks in her eyes. He sees that Nei and Mars live and that he has to die in order for them to grow more. Thats’s why he jumps, he does love Mars and it’s people and therefore he sacrifices himself...
Layla is the victim of the decision by the Original dozen that also heralded the end of children being born on Mars. As such she is a child of the end of Mars (a population without children would eventually grow old and die). Besides that she is the one that lives to Avenge the death of the innocent people aboard that ship, thereby also cleaning the guilt of the people of Mars. It is quite possible that she is indeed a ghost. Not in a Casper the friendly ghost way, but like in that she has been granted life temporarily so she can Avenge their deaths. This kind of Avenging ghosts are more common in literature and movies. That way it would be her own grave among the other graves of the people onboard that ship. It's possible she is a ghost, but not necessary i think.
As for her (maniacle) laughing, i guess that was just a sort of cynical reaction to her realisation that she had protected life when she had just wanted to kill those people. (it would be even more ironical if she had done this as a ghost)
That red moon and Layla seem to have a somewhat similar connection as Nei and Mars have. The red moon and Layla first come to mars at the same time (as a result of the destruction of earth?) and the 2nd coming of that moon also coincides with Layla’s arrival and both are expected to bring death. Though as Layla doesn’t come there anymore just to kill those people but instead to save Nei, so does the moon change from going to put Mars to a destructive end into being the catalyst for the water inside Mars to submerge ard bring life. I guess that is most apparent in that Barbarian with the blond hair (Garcia) who reacts quite surprised to that sunlight: “hey, that wasn’t supposed to happen...”. Again that would mean that a ‘killer of life’ changed into a protector of life, just like Layla.
Well, this is just my opinion about this anime. I must say that reading your takes on this anime helped me understanding it a lot
[edit]Whoa, it's 7 AM here and i seriously need to get some sleep... [/edit]