Yes, you can get free if you already purchased VRF Plus 

The original design of VRF Plus is for series production done in the Blender pipeline, especially for rigging and animation, but we never tested to do that, if you want to export to other software outside Blender, we didn't guarantee for that pipeline, but it should be can ;)

Yes, The construction of rigging is a very common standard in Blender 2.9 or higher, but don't joint the Armature Object created from VRF Plus to other Armature Object that you created, it will break the ID and VRF Plus Animator Addon can't access the data, please contact developer if you still confused ;)

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Yes, VRF Plus already tasted real production for years and still continues to be used in production

No, VRF Plus original code for Blender 2.9 and 3.x , using all-new feature of Blender 2.9, 3.x and 4.x from code API or new method that blender given
but you can still try our old product here