[1]
Inside SkeletonAnimator.cs

Around line 120:

animationTable[GetAnimationClipNameHashCode(info.clip)].Mix(skeleton, Mathf.Max(0, time - deltaTime), time, stateInfo.loop, null, weight);

All these lastTime were wrong, you made it Mathf.Max(0, time-deltaTime), that is assuming the speed scale of animation is 1, if anyone change the animator state speed in editor or entire animator speed in runtime, the lastTime parameter then calculated a wrong value. It is working fine because spine-csharp runtime didn't actually using the 'lastTime' parameter to interpolate timeline values. But it's still a mistake. You probably want to do something with this.

[2]
SkeletonUtilityBone.cs

zPosition has no use or something? I don't understand.
If you have the time please add some tooltips to these fields, that could be more friendly to newbies.

Thank you for doing so much effort to spine unity3d runtime, provided us a powerful, easy to use runtime, with a lot of cool features (like foot constraint, ghosting), and great editor, and thank you for those tutorial videos, that really helped me a lot.