Esoteric Software

[Harald]

1. How I play a animation exactly once in runtime.

Please see the documentation:
spine-unity Runtime Documentation: Setting Animations
And please take a look at the example scenes that come with the spine-unity runtime, in directory Spine Examples.
There is also a video available about the spine-unity example scenes: https://www.youtube.com/watch?v=DxDZtTK2nlE

The full runtime API reference can be found here:
AnimationState Methods

2. Is it a must to keep skeletongrahics GameObject active before Play in unity to load/work?

No. You can always issue the respective initialization calls that would be called by Awake yourself, e.g. skeletonGraphic.Initialize(false); and subsequently.