Thanks for the information in your two replies. I haven't researched the Dunscombes as thoroughly and impressively as you. I've received all email posting notifications from GenForum and posted my original query also on Ancestry and Rootsweb (in the Bermuda, Dunscombe and Ogden boards - as here), but have received only these replies.
It's tempting to abandon any definitive attempt to sort out this complex Dunscombe family, particularly the many named Thomas, but your finding that Samuel had three wives, not two - with four of Samuel's children born to the third, Elizabeth Pitt, brings new perspective.
Julia E. Mercer's book, "Bermuda Settlers of the 17th Century," identifies only Elizabeth Burgess and Abigail (no surname) as Samuel's wives - mentioning that the latter was "buried in 1684" and that their son, Nathaniel, was baptized on Oct. 12, 1679. It shows Samuel (twice), Joseph, Martha, Hannah, Benjamin and Jonathan as children of Samuel Dunscombe and Elizabeth Burgess (without mention of Thomas). Of course, Mercer's account could be inaccurate or incomplete; there are inconsistencies in her account.
The existence of a third marriage implies that my subject of interest, the elusive Susannah Dunscombe, if she was indeed a daughter of Samuel (most likely by one of his first two wives, given her approximate marriage date) was excluded from the will because she received her due before leaving Bermuda, as you suggest, or because her whereabouts (then in New York) was somehow unknown to Samuel when he wrote his 1706 will.