Just kidding ;-)There were actually many more Reuben Allens than the three you mention, but for the ones you do mention, I think I can clear up the relationships.Warning I am only quoting from Davis & Dick's 1984 book, "Allens: Quakers of the Shenandoah," but it appears quite well documented.
Here's what I have worked out:
1) The Mary you mention above is the MOTHER of the first Reuben you mention, but in fact her husband was also named Reuben, and your Reuben #1 is their child (let's call him Reuben II), and Ingabor is his wife (widow).This father Reuben was the brother of the Benjamin in your evidence.Evidence: Petition for letters of administration made by Reuben Allen as "eldest son" of Reuben; see p. 10 of Allen book.
2) Deborah Allen m. Jacob Rambo is suggested by the authors as a possible but unconfirmed daughter of this OLDEST Reuben, but these authors cannot offer any firm evidence either, see pp. 422-423.
3) Your second Reuben I can't identify, unless he is Reuben I.Do you know what the death date is in reference to (a mention in another's will or something like that - whose?)
4) Your third Reuben appears to be Reuben III, son of Reuben II and Ingaborg.Although the Allen ancestors up to that point had been Quaker, Reuben III was not raised a Quaker and appears to have served in the Continental Army and died of his wounds in 1779 (Shen Co. Will Bk A 280-81, naming his wife Lydia and 5 kids).Lydia was his first cousin through Reuben I's daughter Mary.
The Davis & Dick index lists no fewer than 10 Reuben Allens, which doesn't even count the Reubens with different surnames descended through daughters.It appears to be a quite well documented book and I would be happy to do lookups for folks if they don't otherwise have access.