...which is really his left arm. ^^;; I guess the akuma have been mostly forgotten, for now (in this chapter arc). I really hope we'll see them again - and yeah, that would give him an emotional connection even if the obvious isn't true (if Mana's the Noah, and gave up power, who's the most important person to him...).
The entire Order is dark - I was actually reminded of it when reading Claymore. They almost make some of the Noah seem like nice, normal people. So far, only Allen's and Rinari's Innocences have done weird things - and yeah, they're probably/could be the Heart. If something happened to Rabi, for example - maximum invocation? - he probably wouldn't be in the same situation.
In a combination of the two thoughts - Innocence and Dark Matter are in complete opposition, which is why they harm each other, but aren't they nearly the same thing? So perhaps Innocence can be corrupted at some point...
Probably, at some point we will be told that everyone that has this kind of power (Exorcists, Noahs, etc...) is part of a "heart". Once they manifest that part of them, their true nature will come out. If heart is been taken here (the plot) as emotions, or a reflection of the soul's nature, then it cannot be something good all of the time. It will be either good, or bad, depending who's manifesting the power. That leads me to think that the minions of the Earl may end up having some sort of heart-like power later.
It'd be interesting if the plot does not follow absolutes, as in, the main guys are 100% good, and the antagonists are 100% evil. I'd like things to be a bit more relative, since no one, for all I know, can be 100% good.
There isn't something like 100% good and 100% evil. It's just like that stupid division of good and evil as black and white. If a manga end up like this I rather not read this because it's kindergarten level. It's not the real world. So it's pretty illogical. I'm positive Allen won't end up like that. ^.~
D.Gray-Man was moving far away from absolutes before this chapter arc, though (and I don't doubt it will keep going that way afterwards). From the semi-spoilers(?) I've read on Mangahelpers, there might be some flashbacks to the Order...? Or maybe those are predictions? Either way, we have what appear to be Bad Intentions from a Good Guy. (Well, it's Rabi fighting zombie friends...)
I'm pretty sure the Heart is an actual Innocence in this case, though. I mean, the idea of everyone being part of it is probably partially right - the Heart is the... what's the word, the core? of all Innocence - but there definitely seems to be a difference between normal Innocence and THE Heart (the Heart is destroyed --> all Innocence is destroyed. If all Innocence were equally powerful, why would that idea exist?).