I have a theory that Dipper is possessed but not necessarily by Bill Cipher. I believe that Dipper may be possessed by one of Bill's deciphers or friends. Or that he might be possessed by a ghost or Spector of some sort. This would go along with the fact that he hasn't been acting all that strange. The style of behavior doesn't fit our usual thought of how Bill acts when he posesses someone. In times before, when Bill has possessed Dipper, he was always very obvious about his possessions and made his presents known. Some of his behaviors include slamming his (Dipper's) arm in a drawer full of forks and falling down the stairs. Also, Dipper's eyes were yellow and his pupils were elongated when Bill possessed Dipper. And in all seriousness, have we, the Gravity Falls Fandom, ever known Bill Cipher to be subtle about his presence? No because he loves to make his presence known and obvious. And when the shapeshifter showed Dipper the final form he would take, how would the shapeshifter know that. Did it ever say he could see into the future? No, it doesn't. So the shapeshifter could have been lying to scare Dipper. The tapestry in Northwest Mansion Mystery could be foreshadowing when Bill takes over gravity Falls. It shows people bowing to Bill as if he were their leader, so for all we know, the tapestry is an family heirloom and the Northwests' just don't know what it represents.