uh. they're all emblem heartless dude, pumped out of a machine rather than kingdom hearts' loins. those are the purebloods, where true nobodies are made from, when somebody loses their heart it becomes a heartless and their body becomes a nobody.
It doesn't matter if it's a Pureblood or an Emblem, the conditions for spawning a Nobody in the first place are the same.
Only strong-hearted beings produce a Nobody when they lose their heart to Darkness. It doesn't matter if that loss is brought about by succumbing to their own Darkness (resulting in a Pureblood) or if the heart was forcibly taken/stolen by an Emblem.
It does not always result in a Nobody, not every Heartless has a corresponding Nobody.
Fair enough. Memories are a component the heart, among the many others like Light, Darkness, and the other unknown elements. Speaking of, is there any credence to what Dr. Finkelstein says to what makes a heart too, like Pulse, Surprise, Hope and Fear etc., since he did mention "Memory" and he got that right? Memory also extends to the body and soul, since Nobodies can act as if to have feelings while waiting to nurture a fledgling heart.
Since Nobodies could also nurture a fledgling heart, wouldn't that mean the "Special Cases" Nobodies like the Replicas and you-know-who just got a little less special, since they now have a foundation to actually have feelings?
Surprise, Hope and Fear are emotions themselves and thus originate from memories, as Xemnas states "rub a few memories together and you get a spark of emotion, a feeling". As for "Pulse", that was most likely Finkelstein mixing up the actual organ "heart" , which is called "Shinzou" in japanese, with the spiritual "heart" KH mostly talks about, which is called "Kokoro" and can also mean mind/spirit/essence.
That is something we do not know. We don't know where the memories that Nobodies use to form a personality and pretend to have feelings are stored, considering it is just a lifeforce battery feeding the body with energy though, I personally highly doubt that it is the KH-variant of the "soul".
Everything can have/nuture a heart, that was the main in-universe reason why they introduced a Data-Sora in Re: Coded, to show that he grows a heart. Tron is another example for this, DDD was only the first instance to blatantly point it out instead of referring to it in a roundabout way.
Replicas are implied to have an (empty) heart from the start though which is still different from the run-of-the-mill normal Nobody while Roxas, Xemnas and Naminé are special due to several other reasons, the capacity for true emotions isn't the only one and in Roxas' case specifically, the speciality comes more from how fast he develops as he's the youngest of all the Nobodies, yet thanks to his connection and co-existing with Sora as well as the presence of Ven's heart, he develops faster than all others.