Re: upper register
It could be the mouthpiece reed combination. If you have a very closed mouthpiece with a soft reed it will have a tendency to close off as you bite. As you go into the upper register on soprano, the tendency is to bite. However, you should try to avoid this and make sure you maintain an even embouchure throughout the entire range. That being said, you may also want to try going up in strength a bit in reeds to see if that makes a difference. Also, try a few other mouthpieces and see if you have the same issues.
Its also true that some horns close off in the palm keys more so than others. I've played several Selmer series II's that have this problem. Its also possible that its a key venting issue and you may want to have a competent tech take a quick look at your horn.