Humanoid Erratic Twisting at end of clip regardless of where I set the end of the clip. How do I avoid this or fix the problem?

Here is a video demonstrating the problem. https://youtu.be/Ri_WJWX8fe4

You'll notice that the animation itself does not contain twisting in generic mode, but in humanoid mode there is erratic twisting in the left arm at the end. Then I truncate the animation clip to before the twisting starts twice more and each time twisting happens at the end. There was no twisting there at that point in the animation clip when it was longer, why would it remain?

This is a similar question: http://answers.unity3d.com/questions/847346/humanoid-rig-treats-animations-differently-to-gene.html

The problem is the same... twisting of a joint that needs to hold something. But my question is specifically why I can't just cut that part off?

We also use a weapon attachment bone, and one of the proposed solutions is to not do that, but as you can see in my video its not the weapon in his hand twisting (he actually isn't holding one in the video) but his entire wrist and forearm.

Solving this problem probably won't solve the greater problem of semi-random erratic movement variations from what the animator animated, but until now I haven't even really been sure what to ask. I tried all the solutions I've read, including gimball lock, using masks, tweaking the muscle settings to allow for greater range of movement, imposing strict rules on the animators about how far a joint can be rotated between two frames, and more. But we still get inaccuracies and erratic movement. We haven't been able to get consistent results by with any one solution attempt. A tweak in the animation like exporting things as individual animations rather than as a set and the cutting them in unity worked in one case, but not in another. Its excruciatingly frustrating.

Help... please...

Again, at the very least I need an answer to my question: "why can't truncate twisting from the end of a clip?"

Ideally though... What do I do to make all the humanoid stuff behave itself and be usable in an actual real-world situation? Games aren't made with generic human rigs and perfectly realistic human animations. There are many kinds of humanoids with many kinds of unique requirements and game animations often need to be in the realm of impossible to look fun.