The Shadow "knows"

[Crystalion]

I guess the real question is, will spreading my skills out to help my shadow master really benefit her enough to justify not spending those points in skills I would be using myself?

A reasonable question. But from my (inexpert) point of view, it seems to me that this question is hard to answer without understanding the ai of the shadow master pretty thoroughly.

Even then, her equip (random) rolls and (random) combat decisions will make her effectiveness vary a lot.

The bane of the Warrior is that you have to tell them what to do (and this cuts into what you want to do, most likely). The blessing of the Warrior is that they (often) do what you want them to do.

For all I know, however, the behavior of the Master can be influenced.

You certainly influence shadow target selection by ranged/melee attacks (for example, what you target with Blade Fury is very likely to become the focus of your shadow's anger the next time she makes a targetting decision).

A great deal of "investigation" needs to be done here in practice (and, as I say, I am inexpert).

Good luck with your build. Glad the post helped shed some light for you.