Quote - YES. you will get texture stretching on the collars-shoulders when you lower the arms.

I don't know much about modelling or UV mapping, but it seems to me that just because you model the mesh in a t-pose it does not mean that you have to UV map the mesh whilst the figure is in a t-pose. As I understand it the mapping is tied to the vertices, no to any spatial coordinates. You should be able to rig the figure, export the mesh in any desired pose, map that posed mesh, then apply the mapping to the t-pose version of the mesh. I'll stand to be corrected on this by someone who understands more than I do about it, but that's how it seems to me.

You could for example pose the figure with the eye lids half closed, the mouth part open, the arms down 45°, and the knees slightly bent. Export and map that, then apply the map to the base mesh. No?