I've come up against the same issue with this: William is well documented, there's no getting out of him being at the Battale of Durazzo in 1081. So let's say he's about 20 at that time. That would seem to make him way too old to be Petronilla's father if she is born in 1149, and that is date is positively confirmed (?). Also, Mabille Hauteville is his wife, mentioned in several sources, that's why he's at her father's bedside when he dies, as one of the family, and that would more or less fit with her being Bohemonds sister age wise, as well as Williams wife.
These dates do not seem to fit as well with Alice Beaumont being his wife. However, if William and Mabille had a son William II, this would fit very well as being both Petronilla's father and Alice's husband. It's just a theory, but it would make the dates plausible.
