The period photos are great. Our unit uses the straight up position for Support Arms. It should be noted though, that this position works best when stationary, or at common time. The faster the pace you march at, the harder it is to keep the musket up. Even one of the period photos shows moving soldiers and at least two of the muskets are leaning backward.
Part of the problem is that if you place your left hand too high up, it forces the musket to tilt back, as the balance is harder to maintain. The main advantage of having your hand higher up, is to grab the cartridge box belt and take some of the weight off the left arm. Period soldiers did this as well.
On the Right Shoulder Shift, what may be required when marching or standing, will look different when at the double quick step. The angle seems to be the most accurate per the manuals, yet I think when soldiers are soldier- to- shoulder, the right arm and elbow may get shoved in and make for a more vertical look. Regardless, our unit uses the earlier manual with the angle. One problem I have seen, is that some reenactors try to maintain elbow contact and will stretch out their right elbow more than it should, which forces the musket to a greater angle than is period, and is potentially dangerous to the rear rank.
1st New Mexico