Thanks for the feedback and for looking into it. I cross posted this response to the C3 Forum, just to show the love for Spine to hopefully move things along.
I understand the comment about tex tris and Mesh/FFD is an essential part of Spine, so obviously you need any necessary support for Mesh before a runtime makes sense. Did the C2 API support this? (See below on C2 Spine plugin.)
Also, I imagine you are well aware, but there is an unofficial/unmaintained Spine Plugin for C2 the notes say that it supported Mesh/FFD (with Github source). I never tried it, it looks like development has stopped on the github.
Here's a C2 demo of the plugin it looks lilke FFD/Mesh is supported (isn't the tail animation using FFD/Mesh?)
On the spritesheet side, I am probably out of my depth in terms of Spine/Spriter details, but in the Spriter/Construct flow, I don't remember anything about manually doing spritesheets, the flow felt pretty automatic. Here's the tutorial I used in the past, perhaps you can check if this manual method was needed. You just need to watch the first few minutes for the export/import stage (the rest is about controlling the Spriter plugin animations, etc. once it has been imported.)
Spriter C2 Import Video
Some active discussion on C3 and Draw plugins (Spriter/Spine/Creature) going on on the C3 forum, I imagine they could use input from the Spine leads about what's needed for Spine.
https://www.scirra.com/forum/drawing-plugins-now-supported-in-r68_t197970?&start=10
