The reasons will work, then, but the powers sound a bit OP.  Remember, they just got their powers, and will therefore be far weaker than they can be.  I also don't see how "flight" fits in, either.

Having them tied to the time of day works, and I don't have a problem with him getting more/less powerful as the time of day changes, but "He can create nonsentient allies/Make virtually anything he wishes" maybe even the "making mounts" things need to have limits placed on them.  Otherwise you could make an army of squirrels, bears, etc, or create balls of shadow that can destroy cities, or a dragon mount.

Darkness is a hard power to balance.  The only time I've used it I listed the exact powers I had and what the limits of them were (example: envelop one person in shadow, blind one person temporarily, beams of darkness that can knock people backward, infecting them with Darkness that makes them sick, absorbing darkness for energy(lightens the area and weakens me), create "shadow fog" (very limited area))