yeah but no but I’m getting confused here
A lot of the fan response I’m seeing here seems to be based on the assumption that Stanley, as great-uncle Stanford’s brother, must be Mabel and Dipper’s grandfather. But, you know, not necessarily? They could have another brother who is the twins’ grandpa. Having him be their grandfather means you also need to account for their grandmother, and who she is and what became of her, and what she knew, if anything, about Stanley’s disappearance, which, to do it properly, means establishing two new characters instead of one, and unlike Stanley, this putative grandmother has been completely unforeshadowed. That seems like a narrative complication that doesn’t fit the build-up we’ve had.
I just think it’s more likely that Stanley is another grunkle. You’re allowed more than one. A good story with multiple mysterious grunkles is The Haunting by Margaret Mahy. Actually, I think both Dipper and Mabel would really enjoy that book, just from somewhat different angles.