I have had a look at Cox burials at St Mary, Buckland to see if they give age details which may determine whether any of the Williams were of an age to have married both Rose and Hannah. Unfortunately no ages on burial were recorded in Buckland registers prior to 1812.
A Hannah Cox was buried 30 May 1788 and a Rose Cox 14 April 1796. I cannot see a burial for a William Cox at St Mary in 1788, which could indicate that if there was a death that year he was a non conformist, although if he was the husband of either Rose or Hannah why was his wife not buried in the same place. The Rose burial in 1796 may not have been his wife though and there is no other burial at St Mary. The only Williams that were buried in St Mary were 8 Nov 1803, 18 Jan 1811 and 24 April 1814. The latter was age 70 so he could have been the husband of either or both women.
If you find a photocopy of the original marriage register for Rose's marriage it should tell you if that William was a bachelor or widower at the time. If he was a bachelor then he could not have been the husband of Hannah, although if he was a widower does not confirm that he had previously married Hannah. Likewise a check of the Hannah's marriage, if that William was a widower then it is unlikely although not impossible that he had a third marriage 26 years after the second.