Hi Viv. I think I can help a little. While doing the Marown Kewleys I chased down a bunch of others and this is one of them. I believe your William K (h/o Isabel Clague) b ca 1690 of Kerroo Doo is the son of John Kewley and Margaret Corkill of Black Quarter Onchan. I looked up Kerroo Doo in the Place Names of Onchan in the Manx Notebook, and it says that in 1703 Man. roll it was Black Quarter ('black quarterland'). So they are one and the same.

John Kewley was buried 28 Jun 1710 Onchan and wife Margaret 14 Mar 1734/35 as Margaret McYlroy als Corkill (likely remarried but I didn't follow up). Both had wills (below).

Children, all Onchan:
1. Isabel 23 Apr 1688
2. William abt 1690. Of Onchan, eldest son, in mother's 1734 will.
3. Margaret 3 Dec 1693 "Margrett daughter of Jon Kewley black qr".
4. John 24 May 1699 "Jon son of Jon Kewley (Blackqr)"
5. Gilbert 8 Nov 1702.

My notes for John Kewley's 1710 will, seen on Jim Smith's site:
Will: KEWLEY, John 1710-2 Onc E w 0991649 (Mannincloud - transcribed on Jim Smith's site)
Conchan 1710 LWT John Kewley buried 28 June last.
Son Wm crop, husbandry, sheep, etc
Two children Gilbert & John 30s betwixt them
Wife Margt Kewley
Wife & son Wm child at age
Witn: Jony Christian Silv. Quyne
Pledges: Wm Cowen & Wm Kewley
June 12, 1727 John being dead, Gilbert to be paid from his mother the 30s bequethed them both. Witn: Gilbert Kewley his mark X.

From MNB Will Summaries, Braddan:
MNB Braddan:
1735-1 A 6 McYLROY Margaret CORKILL d 30 Dec [1734];ch Wm Kewley(eldest son -Onchan), Gilbt(+ wife, Kk Braddan) exor; gch Robt Kewley, Esther Kewley[assumed gdau];1739: Robt Kewley acks uncle Gilbt;

I know quite a bit about Gilbert Kewley and Joney Corlett (married 21 Mar 1725/26 Braddan) because I had to sort out their son Robert from my ancestor Robert (b ca 1737), parallel families of Braddan. Their children, all Braddan:
1. Esther 12 Apr 1726. Likely the person mentioned in grandmother's 1734 will.
2. John 1 Oct 1727, has 1757 will (see MNB for extract)
3. Richard abt 1729 (Braddan burial 1735: "Richd Son of Gilbt Kewley Ap 4th"
4. Mary abt 1730, md James Corrin 22 Nov 1748, issue. Both buried 1789 Braddan with wills on MNB.
5. Robert 10 Oct 1731. Md Ann Clark 12 Jul 1760, 8 children. Robert was a joiner of Douglas, bd 26 Aug 1804 (will). Ann bd 8 Aug 1806 (no will)
6. Gilbert bp 7 Apr 1734.

The bit at the end of Margaret's 1735 will "gch Robt Kewley, Esther Kewley[assumed gdau];1739: Robt Kewley acks uncle Gilbt" is a big clue that we're on the right track here. Gilbert's son Robert is only 8yo in 1739 (and not born when grandmother died), but William's Robert was about 23 and he'd be the one referring to "Uncle Gilbt".

I have to send this now as I have to go out. I'll add though that you and I are 4th cousins through our Gelling ancestors and probably through the Scarffe one's too. But not through the Kewleys.

BTW, the "male 10.04.1720" is John. FS Baptism Onchan 1720: "Jo'n, Son of Wm Kewley baptized Ap;ril ye 10th"