It was year or more ago when I talked about that we need Animation, Skins, Events etc. Views and we finally come to the point that it is really needed. :-)
I understand that we can not get rid of Setup and Animation modes and I also didn't mean to have side by side Animation View and Setup View. They were meant to be just views in their respective mode.
Detachable views are must the same as saving layouts. There is no way around it. :-)
The trick with Unicode does not work in on 5k monitor. The text is "terrible". I don't understand much why you disregard SDF solution since the resulted render of sdf should be clean and nice no matter how big or small the font size is.
Regarding UI graphics. I would propose a deal here. You give us two licenses of Spine Pro and we will illustrate all missing graphics in illustrator and prepare it so that it is easily exportable for your needs. We can use for it our IntegratorTool2D which is really handy tool for things like this. Sorry again here on 5K the icons are simply terrible I dare to say unacceptable.
Back to Animations, Events etc views. I don't see combo box for selected skelton as good idea. List skeletons like you list them in Tree. It is much more quicker to navigate through them and if needed select multiple animations, events etc. from different skeletons.
I will write more about views once I sort them out in my head. :-) But generally there are Tree Views ( Animations, Events, Constraints, Draw Order, Skinds, Events, Images, Audios ) and there are Tool Views. Do you feel it same?
So please let me know with the graphics. As I wrote for 2 Spines Pro licenses we can put them into pro state. ( low res, high res )
Also the trick with stretching spine over two monitors will likely only work if you have same resolution monitors I guess. Does not work here with 5k and 1920x1080. :-(