Thank you for your detailled writeup.
In general it would be much easier to follow the discussion if you post the text and images here on the forum, instead of attaching a link to a word document.
Also, when showing code, please do not make a screenshot of it, but post it in text form (the forum offers code formatting for this reason), so that in case anyone wants to replicate the behaviour, she can copy and paste it easily.
Now regarding your problem:
I need to alter the “_Cull” parameter in the Material.shader according to the direction the character is facing (left or right).
[..]
My character is in 2.5D and I cannot use the Cuffoff method, as it will cause the image to display incorrectly. I can only use the CullFront/CullBack method.
Why can't you disable culling?
Regarding setting material properties on a single object instance, it is recommended to use MaterialPropertyBlocks instead.
You can have a look at the spine-unity documentation section here:
spine-unity Runtime Documentation: Changing Materials Per Instance
How do you rotate or flip the character around?
Please note: If you only need to flip the character from left to right, then the simple way in the Spine runtimes is to set skeleton.ScaleX to -1. If you really need a rotation animation around the vertical axis, then the scale solution will of course not be sufficient.
