I know the recedived view is that, as Mr. Williams stated,
Elizabeth Mure, wife of Robert II, was daughter of Sir Adam
Mure of Rowallan and his wife Jonet Mure, heiress of
Polkellie. However, several years ago Andrew MacEwen (who
is highly knowledgeable about these Scottish lines) told me
that in fact Sir Adam Mure was of Polkellie, not Rowallan,
and that his wife was Jonet Cunningham, widow of Hugh de
Hutscom. Andrew said this is proved by a papal dispensation
of 1322. I'm afraid I can't find the citation, if he gave
me one.
I don't know what Andrew's view is of the purported Montgomery-
Mure descent. The account at SP 3:421-427 suggests that the
Montgomerys are pretty speculative down to c. 1360, and differs
substantially from the account at ES III/4:640.