Pip, The first obstacle is that no marriage record has yet been located for Alexander Underwood, other than to his last wife Sarah. He had no children from that final marriage. Quaker Meeting records do show a wife named Jane who appears to have been mother of all his children. There is some controversy as to whether there may have been another wife prior to his marriage to Jane. Sources are needed other than Ancestry trees and LDS files.
All seven of his sons are named in his will. The daughters appear to be where most of the confusion lies. Only four of them are named in his will. These were Mary Wells, Ann Frazier, Jane Smith, Ruth Morthland.
I've made some comments on several of the children's profiles with notes of changes that could be made. Hope that helps you in your search.