Hi, it's about three years since I last tried to pin down David Low's origins, but everything you have written matches what I have in my notes about his possible/likely family. John Dewar, his wife, and daughter Janet (Jessie) are also all buried in Forfar, whilst Alexander was living at Gowanbank in 1871 and his death is recorded at Brechin in 1899.
Any evidence as to David Low's actually family is always going to be speculative as his first appearance in the records found so far was in 1855. Where he was in 1841 and 1851, and under which name, is still unknown. He may have been born David Dewar, as per his marriage certificate, and raised by another family as David Low (or the other way round), or, as I assumed, he took the name "Low" from Low Street, where his wife's family lived. I have found other examples of adopted people using one name on official documents and another in everyday life, so it's possible that John Dewar was his natural father and that David is under another name in 1841, and at sea in 1851, making him virtually impossible to trace.
On the 1861 Census Crew Lists, the requirement was to give the "town or parish where born", and I thought it reasonable for David Low to have given Montrose (rather than Forfar or Brechin) as it was the nearest port town to his possible place of upbringing (if Fern/Deuchar), and was probably the one from which he first went to sea. He may also have lived there as a child, but who knows? You certainly can't take anything on a census form at face value, so I don't think it odd for a different place to have been given (I've got scores of examples on my tree, some giving a different place of birth on every census!).
Something else of note is that most of David Low's children were named after members of his wife's family - a very rare thing in NE England - which leads me to think that he either didn't know his own family, or wanted to remove himself from them, for whatever reason (illegitimacy?). If that's the case, he did a very good job of it!
As for other links from David Low to the Dewars of Deuchar, there aren't any direct ones that I know of: I got there by a process of elimination, tracing the other possible David Dewars backward and forward and ruling them out because they were definitely not the one who went to Sunderland and Hartlepool, leaving John Dewar of Deuchar as the most likely father (always assuming, of course, that David Low's real father actually was a John Dewar, as named in the marriage certificate). I could well be wrong, but I do state in the notes on my tree that the relationship is still unproven, and may always be so.