These are some very tricky arm poses. Proko’s Skelly app could be a good resource for you to use. I think it’s $7 in the App Store. A good price for how useful it can be. I took a few screen shots of Skelly to give you something to reference. It looks like you understand the arms pretty well. The scapula seem a bit awkward to me, but that’s all. And you were correct in identifying the Teres Major. This guy has big shoulder muscles. You dont normally see them quite that much. Hope this helps.

This is a very difficult position. I think you did great. I think a bit less of the front plane of the face would be showing, so I would raise the chin and corners of the mandible up a bit, and also raise the shoulders to match. The only other option I can think of is the external jugular vein as a possibility for that piece of anatomy, since it is overlapping the sternocleidomastoid and wrapping inwards.