No you can never change your shaman in the campaign games, there may be a cheat code but it will probably screw up your game

Yes, you can. You have to go into the game's code and switch the sprite for yo9ur shaman with the sprite of one of the other shamans. I don't remember how or else I would say how to.

Yes you can, but just to inform my answer is to above. BUT DONT TRY IT. The populous
system has picked up on the cheat code and disabled it. It will muck up your populous.