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
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:
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.