OMG OMG OMG ALEX NO DEAR GOD NO<----- NO! WHEN I HEARD BILL SAY THAT THE FIRST THING I THOUGHT WAS "if I were Bill I would choose Soos. The one man who's an honorary Pine, trusted by all of them, even Ford. Tricking him into making a deal with me? No problem. Guy would sell his soul to the devil if he thought it would save the twins. Remember 'sorry Mr. Pines but I have a new mission now. Protecting these kids!'" I REALLY HOPE I'M WRONG!
NO LISTEN TO ME. He's always lost at this game,ALWAYS. It had to have been him Bill was going to pick, you know it as. Now just think about the nightmares he has if this, OH GOSH SOMEONE HELP THIS KID!!!!
Edit: This post was made before we knew for sure that Stanley/Stanford was the author, or that he even existed. As such, this theory does not line up with actual events in Gravity Falls. It is super cool though!