§ Issabel's Will of 1684 possibly names a se[cond] son - not necessarily William?
§ She sets the proviso "....if ALL the children die UNDERAGE then husband Gilbert to have all her...? + 10/- to brother Wm Keggeen...."
this would imply that all her then extant children were born after 1669/70.
§ In Gilbert's Will of 1690, "Tho Skillicorn Supvisor of ye CHILD AT AGE on ye Fathers side and ye Ant & uncle on ye mother’s side viz: Wm & Margt Kegeen" - This child was thus born not after 1676 - but possibly "of tender years" [as they say]. Does this imply that the other 3 children are over-age? If so all the children must be from marriage no 1.
"Thomas Skillicorn supervisor....on ye father's side" MIGHT imply that Gilbert's second wife was a sibling of Thomas [the usual relationship of a supervisor], but he is not named as an uncle as is Wm Kegeen.
§ A Margaret Harrison was a witness to Issabel's Will - a possible Harrison connection, but can't see it at present.

I think the most likely hypothesis is that all three of Joney's siblings died before 1700. If William was from the 2nd marriage, and the "blood line " was from the Keggeens, then he could not inherit anyway. Joney returned from Ireland to claim her inheritance; she had married Christopher Bell by 1703 [? he may have been b.1676 Malew s/o John]. They sold up in 1709 to John S, but he had to forfeit to the Quilleashes. Joney & Xtopher had at least three children: Margaret [b.?], Ann [1712 Rushen] & Christopher [1717 Rushen]. Christopher snr d.1732 Rus [Will mnb] and Joney married John Crebbin 1732-11-12 Rus. Joney d.1741 Rus, her Will [mnb] names her 3 children.

The question of land ownership remains crucial: 1703 Abbeylands in Skinscoe: "Jony the daughter of Gilbt Callin and Christo Bell her husband for half a quarter of land called Ballarragh of 5s 3d rent compounded in 1666 by Gilbt Callin and fine then was 52s 6d years determined pays now 02 12 06"
Could Gilbert Callin "compound" for the land in 1666 if it was his wife who had inherited it?
Alternatively "Thomas Skillicorn on ye father's side" might imply that the connection is further back in the Skillicorn line. This Thomas Skillicorn could not have been the son of Gilbert Skillicorn - he would have been only 10yrs old in 1690.
Rgds
JohnL