Will the stone disappear?
I just tested this out. You cannot press "B" if you used a stone on the Pokemon. I used a thunderstone on my Eevee and pressed "B" like crazy and nothing happened. So to answer your question it is impossible to press "B" while using a stone. My guess would be it would dissapear if you could press "B".
Yes,like all evolutionary items, it disappears after being used for evolution. So basically, only one Pokemon can be evolved with one stone.