Classic 2D games (and even some modern ones like Rayman Origins) use specifically animated transition-animations that minimize automatic mixing in favor of transitions that the animator designs. Games like Ori have no mixing at all since they use pre-rendered frames, but if you watch videos of it, the animations flow very well into each other.

Like many animation tasks, this can be a little painstaking, especially if you aren't used to the task.

Game animators will also know to use large, fast movements that ease into the desired poses in order to make sudden transitions into animations look visually believable even if it's not logically believable.

As for the arm doing a 360 during the mix duration, the spine runtimes currently have code in place that should prevent this, but if you arrange and animate your bones in awfully specific ways, it can still happen.

The mixer will always rotate a bone in the direction that will create the smallest movement. This is why you will sometimes have a character's arm twisting the wrong way. Here's an example. The picture below (sorry for the crudely drawn MS paint file) is an arm mixing from one animation to the next. You would want it to rotate counter-clockwise and swing all the way around, but it will end up mixing clockwise because that's the shorter distance, even though a human arm doesn't bend that way.

The only way I've found to avoid this is just to make sure that the path that makes the most sense is also the shortest path.