Re: why pure blood heartless don't release heart's
It comes off as really arbitrary. Why should purebloods act differently from Emblem heartless if Emblems are just copies of purebloods? Even if pureblood heartless are embodiments of the darkness of the heart, it doesn't seem unreasonable to assume that the vast majority of them contain no heart at all. The way I see it, while there are some Pureblood Heartless that possess hearts, the vast majority are formed completely from the darkness inherent to the Realm of Darkness. They could still be said to be "the embodiment of the darkness of the heart," since the Realm of Darkness was created as a result of the darkness in people's hearts: because of the Keyblade War.
Honestly that's cause it is arbitrary. To be honest and fair the entire series is arbitrary. Most of it is left ambiguous and what isn't, isn't explained properly or in a place every fan can see it.
There's no real explanation just stated rules from games and Nomura. Things happen in the series, for the most part, simply cause. (save character driven events such as Xehanort and his schemes)
I just posted a quote stating they have no heart, so that much is a given. Only that they are born from them.
Not that it's not impossible they dont' form naturally in that realm but it's also not stated. They have so far only been stated to be born from the darkness of the heart.
It's why Yen Sid says:
Your past endeavors did prevent an immense effusion of Heartless from
the great darkness; make no mistake about that. However, the Heartless are
darkness made real---and darkness lingers yet in every heart. The Heartless are
fewer. But while darkness exists in a single heart, it will be difficult to
eliminate them.
They come from the heart. They were scarce before Xehanort but not nonexistent. There has always been beings that abused darkness since it's inception into their universe so it's only natural to think a heartless is the final outcome of such a being.
They eventually give in and rise in the dark realm as a heartless just as a nobody, if they are of strong heart, arises in the realm between.
The underlying theme of this series is heart. We see it everything, the title, the enemies, the themes of ties and friendship, Nomura actually saying it bluntly (which is a bit funny since he was asked it in an interview, can only imagine his "dude really?" face).
Memories, ties, heartless (born from darkness of the heart), nobodies (people who've had their hearts stolen), dream eaters (dreams in general born of a heart sleeping), etc.
It all ties back into the heart somehow.