I use maya, but what I learned from a rigger long ago is to set some keys on the joint(s) so that you can scrub through the timeline and see how it deforms. It's easier to see what's going on rather than painting, moving the pose and seeing a problem, going back to paint, repeat... also this solves your "reset pose to original position" question: you key the joint.
shoulders are notoriously difficult though, it's ok. you can also look into other methods like helper joints, binding to geo/muscles as well.
also, your shoulder joint looks a bit too far out, this is why when it bends to the side the shoulder mass seems to move outwards. it's also why when you lift it, it looks like it's bending from the wrong location (cause it is). also, note that it's impossible to rotate up your arm past a certain angle without rotating the clavicle up as well - so the shoulder shouldn't look normal that high up unless you rotate the clavicle up as well.