Yeah, I pretty much came to the same conclusion (biology wise) for female Nobodies, but I'm also thinking "What would Nomura do to make this as unnecessarily complex as possible?" kind of things for a child sired by a Nobody.
Actually, you bring up a valid question, can Nobodies bleed? Or do they have some other form of ichor? Because they're the bodies left behind, I'm sure they do, but now I'm curious.
I'm wondering though, since a human-shaped Nobody is just a person with a strong will, what can be said about lower-ranking ones? Because the threat of being turned into Dusk has been said before. So I wonder just how close to human the high-ranking ones actually are (biologically speaking).
Also yeah, Nobodies do eventually regain their hearts. But I'm talking newborn Nobodies that haven't had a chance yet, so I do wonder if that would have any influence on a child.
PS You didn't answer the most important question! THE SPERM CELLS! Would they have Nobody Emblems on them!? (Probably not because the emblem was created by Xemnas as their logo) BUT STILL!
I cannot answer the first question, because I'm not Nomura's friend/know him personally. However given that I'm well versed in the games, I understand his thought process to a degree, and he tends to shift more to the "make it complex for 'deep' story telling". He likely would add some abstract rules on how the resulting offspring has no emotions (remember, to cry is to have feelings/a heart in KH's universe) right from out of the womb, and given that their entire society is as anti child-friendly as possible (they live in a castle suspended over an ABYSS that's surrounded by a heartless infested city devoid of life) either they likely wouldn't care for it, or if they did they'd likely warp the kid's mind beyond belief. The kid would likely have an identity crisis as it is born to two beings without hearts, yet could/couldn't have one.
I'm going to assume that if they have to do everything that normal humans do (eat, sleep, and use the bathroom) they're not doing it just for pleasure. Thus they have to replenish energy levels in their body by consuming things, which would help with the production and filtration of blood. It's amazing how much those little details fill in the bigger picture. As for what their blood color would be, I'll assume it would be red due to the high amounts of iron in it. (Though if they were to get Atlantica forms, it could be blue, green or even purple!)
The material presented to us in II shows us that the human looking nobodies are VERY rare, which is why there are so few members in the organization to begin with. We're not explicitly told how many worlds are active from KH 1 onwards because that game had a legitimate excuse for the limited amount (the heartless consumed around 85% of them). But if I had to take a guess, they don't show up that often. Since the lower ranking nobodies move in a way that's almost evocative of aliens with how they contort and bend in inhuman ways, it's safe to assume they're similar to cephalopods (no bones to speak of, and is pure muscle) whereas the higher ranking ones have a definitive shape to them.
That's... hard to answer honestly, if both parents don't have hearts then the child likely doesn't have one as well since it didn't lose its heart to darkness.
The sperm cells likely wouldn't have Nobody emblems on them for a very specific reason: It serves no purpose. Most of the biological adaptations that organisms possess either have a function (chromatophores on an octopus to blend in with their environment, for example) or for mating purposes (the bright colorations of male peafowl to attract mates). In this case, the sperm themselves wouldn't have the emblem as they're very simple in terms of their genetic construct, they have no eyes, brains or anything of the sort and as such would have no need for different coloration/markings.