ARE YOU FOR REAL, guys?
For Geoff to be the father of Arieth is an assumption (theory) but not a fact. Ashley and Elena are equally possible. They are still in the house, but they are far away from Geoff, because Elena is comforting Ashley. This means that they are not actively participating in the party and Geoff just hasn't checked the upstairs rooms of the Preps Mansion yet. This is also just a guess (theory), but it can be just as true.
I know Sage said that so she got the guitar from Chad. But from whom did she get Chad? It is unlikely for Chad to give Troy, the guy he loves, a black eye to get the MC's guitar.