Why there are 1 more sticker than needed for colour.
The extras are in case you make a mistake with a sticker. With an extra you have a fall back.
Why white, grey and yellow stickers haven't got a symbol on their brow?
The symbols are to help colorblind or visually impaired players. White, gray, and yellow are easier to see by themselves, but some payers might confuse blue and purple, for example. So the symbols are added differentiators.
Why there isn't a token for the shamans?
You and the other players are the shamans.
Hope these answers help!