I'm pleased to report that I figured it out. Thomas Gelling Miller born about 1810 and who married Agnes Brough in 1839 Lezayre belongs to Door #1 John GELLING & Elinor CRELLIN.
The big clue was the will of Jane Gelling als Kelly 1808 which Jean had researched, and which she sent me details eons ago. In it, Jane mentions a grandson Robert son of Robert and a grandson John son of John, and if this grandson John should die, then to his sister Margaret.
Lo & behold, the two eldest children of John G & Elinor Crellin were John bp 1803 and Margaret bp 1805. Grandmother dies in 1808 and then Thomas arrives in 1810.

I should mention that Jane Gelling als Kelly was the widow of Robert Gelling, Miller, who died in January 1807. They married 22 Sep 1770 Braddan and their eldest was John bap 25 Aug 1771 Marown. We knew he was a co-exec of the 90-yr-old John G of Ballamillaghyn's 1812 will but never could figure out which John Gelling amongst many was him. Now we have it.

I've also found various entries for a John Gelling, Miller which are tantalizingly close to being the 1803 son of John G & Ellinor Crellin, but not totally sure of it:
1841: age 42 at Toxteth Park with wife Dorothy [nee Shimmin md 1827 German] and 3 children. Dorothy's death is in March 1845 index W.Derby.
1851: can't find
1861: age 58 Visitor Wid'r Miller b Braddan visiting William Gelling at Ballagrane Farm Marown. William is a son of Robert G (miller and 2nd son of Robt G & Jane Kelly) & Anne Kelly, and so is a cousin if I got this right.
1871: age 70 Lodger Wid'r Miller b Braddan at Cranyin Ballaugh, home of Thomas 38 and Elizabeth CRAINE 29.

Now we've tied Thomas G b 1810 into the Ballamillaghyn family. A huge piece of the pie comes together.