Molecular alteration relies upon forming the base atoms into a cylindrical shape. The tiny cylinders of air in the blade absorb the kinetic energy, allowing for incredibly brittle materials to gain the flexibility of traditional steel. This is not science fiction, this is something we're doing right now, in order to develop advanced ceramic heat shields on spacecraft, as well as in the creation of tank armor and in Plasma Forges (which need to be resistant to several thousands of degrees celsius). Boron Carbide IS harder than diamond, however it is significantly less tough. It is highly brittle, much like glass, and as such not suitable for anything like a blade. Using this technique however, it is quite possible to create the blade I mentioned.
The blade is not monofilament, as monofilament describes a rope-like structure only a single atom thick. Instead, it utilizes an edge that is only a single atom thick, which lends it it's high cutting capability. The entire structure is thousands of atoms thick, besides for the edge, where it grows progressively thinner until it reaches a single atom in thickness.
Actually, the electromagnet is embedded into the 'sheath' of the blade, placed at the hip of Onikage's suit. The blade itself simply has a small dose of magnetized steel placed within the hilt. This does not interfere with the sturdiness of the blade, and does not require a power source. In fact, I wouldn't even really need to use the entire space of the hilt to make this a feature, realistically.
This blade is completely feasible in our own modern world. With the technology we posses right now, we could build it. The only reason nobody has is because it's a highly ineffective weapon in any realistic combat situation. We don't tend to fight many vampires in the real world.
The exoskeleton is perhaps the most sci-fi thing. At this point in time, we already have direct mental interfaces, and more advanced ones are even now being developed for use with long-range drone combat in the military. In fact, we're so far that mental interfaces are going to become available in the game industry soon enough. However, you're correct in that creating a perfect interface is far beyond our current level of technology. But it doesn't have to be perfect. Just better than a human being's normal reflexes. As anybody who's studied the composition of the human brain would know, our current system for the translation and application of signals is far from perfect, using a series of chemical and hormonal triggers in order to properly translate the signal. A purely electronic method would work much more rapidly, about 50x more quickly than any organic method could hope to achieve.
The batteries are the only unexplained thing. I will honestly admit that I skipped out on the issue of energy containment, and this is most likely the greatest problem facing modern exoskeleton development now. However, we already have batteries capable of lasting multiple years when powering important facilities. I'm pretty sure Kami no kage would need to recharge his suit on a daily basis, based on the application of these batteries as an advanced power source. Of course, more details could be added to this, as I myself have not entirely thought this aspect out.
My friend, any particle has mass, and anything that has mass is incapable of reaching the speed of light. The reason light itself is capable of doing this is because it exists as a wave unless interacting with a Big Particle. Not even the smallest particle could reach the speed of light, because to do so would require an infinite source of energy. Light breaks these rules, due to it's very nature. And if you want your transformation capabilities to extend to subatomic particles charged with inherent energy, then I believe that's something you should have mentioned beforehand.
No offence, but had your character not manipulated things the way he did, this place would be brighter than day. Not only is half of the city on fire at this point, not only is there the constant flashing light of gunfire, but I have several tanks quite literally firing massive beams of fire all over the place. And yet, each of these things are instantly dampened by your apparent capability to generate the absolute darkness you need at-will, even in the face of such great sources of light.
I did not intend for this to become a fight, and I still don't want any offence to be dealt or taken. I simply find that your character has shown to be far more powerful than I believe such a character would be allowed to be.