Multi-skeleton is certainly not broken in the tools and runtime, I am successfully using this right now on version 2.1.27 (OS X), I've only got two skeletons in a file for the human male and human female, but as you can see Spine correctly exports each of their .png, .atlas.txt, and .json.txt files for Unity.
Looking at your error messages, one of your skeletons is referencing a few spt-archer* image files which don't exist, while two other skeletons are referencing missing L-lowerArm, and R-hand1 images ... and another (or one of the previous) is also referencing a missing head2 file.
In order to keep my spine characters organized I use separate .psd files and subfolders. Additionally for Unity I got from the asset store a PSD->PNG tool (called "PSD Layers to PNG Files") to automatically extract all the layers from the .PSD, trim them, and place them in a folder that will be used by Spine.
