I'm fine with the power. Momentum manipulation sounds likes a reasonable enough explanation. I just had an issue with the proposed electron explanation.

By describing the power as momentum manipulation, you avoid sounding like Blake's power. In reality, momentum and kinetic energy are very difficult to relate, because different frames of reference will produce wildly different values for kinetic energy. Simply reducing mention of kinetic energy and replacing it with momentum where appropriate solves the problem with a minimum of modification.
 
 
RE: Ishigo, what you've done there is introduce a strange kind of matter/energy/magic/"celestial element", which interacts with photons. Slowing down photons is a common phenomena. The air itself does it, as does any transparent matter. Of course, to create a shadow, you need to actually stop the photons, either by absorbing them or capturing them. You could trap them, and then turn them into photon molecules. Those are actually a thing. We don't know much about them, and they aren't very big yet, but considering that you are using magic that could form the basis of your physical shadows. But yeah, still pseudoscientific technobabble. Although not quite as ridiculous as the kinetic cores which "captures the unknown energies that are created when an electron simply moves". Electrons don't mystically move on their own. They exist as a probabilistic quantum wave-function over an area of space (typically, an orbital of their resident atom), or they've been pushed by outside means.

Not that I should be expecting scientific realism here. This is fantasy, not hard sci-fi.