You keep making good progress, nice to see…
Ok so here are my thoughts on those points:
Shoulder Joint Placement:
Maybe from the front they could be a little further out and up. But you should test deformation as im being overly nitpicky here.
Spine Joint Curvature:
It doesnt really matter if the fk orients arent in worldspace… because they are in hierarchy anyway. So I would go fully anatomical and try to resemble the spine curvature as accurate as possible. Then if you need fk controls just put them under offset transforms (groups) that get constrained to the prior one and have a second offset group that actually holds the curvature offset underneath those.
Shoulder demo looks cool, would be good if you can show how you are actually placing and driving those joints too.
Yeah thats a common issue. Basically the rotation of the neck vertebrae isnt linear. If you rotate your head 90 degrees to the side, the neck base wont rotate by the factor 90/n (n being the number of neck joints) but it rotates less. Now depending on how you rigged up the neck (I dont know how much control AS gives you in terms of twist interpolation), getting that to behave can be tricky, its either have the joints interpolate correctly and have a straight linear skinning between joints, or just adjust skinning to the way the neck joints twist.
Yeah I see your point, well basically you can do it that way, but it will be quite a tricky task to isolate areas without having their borders double transform or not reach full target state.
I have a little video where I talk about a little joint based face rig I designed in the past months: https://www.youtube.com/watch?v=laG7HQtnrHY
Not saying its an optimal soluiton, but it could give you a little different perspective on stuff like that.
Interesting, you know you can also use different cMuscle nodes that can give you a basic self collision effect right? There is self collision grouping, the cMuscle smartCollide node, you could go with a more transform based setup with keepOut nodes or mesh/matrix collisions on relative joints on problematic areas.
So there is a lot of possibilities on all of these things…
Just keep going and asking questions here, people will be always looking to help you out.