You wrote:
John McCHRISTIAN b. circa 1540
Edward CHRISTIAN b. circa1570
Edward CHRSTIAN b. circa 1600 Douglas, IOM; m abt 1642 to Margaret CALOWE b. circa 1622 of Kirk Maughold, IOM
Will(iam) CHRISTIAN b. circa 1645 Maghould, IOM; m 6 Nov 1666 to Ann QUAYLE
John (William) CHRISTIAN b. ? d. 1705 Kirk Bride, IOM, m 20 Oct 1681 Jane KERRISH
Mark (John) CHRISTIAN bt. 27 Jul 1692; d. 18 Sep 1768; m 5 May 1717 to Joney CROW b.?; d. 9 Feb 1765
Ewan CHRISTIAN bt. 16 Mar 1733; d. 1773; m Eleanor COWELL bt. 21 Oct 1729
William CHRISTIAN bt. 21 Jul 1751; d. 1834; m 10 Jun 1783 to Ann KELLY b. circa 1762; d, 1835
Mark CHRISTIAN bt. 9 Jan 1785; d. 1854; m 19 Nov 1814 to Margaret KISSACK b. circa 1792; d. circa 1870
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I wrote:
Mark Christian & Margaret Kissack married 19 Nov 1814 in Maughold.
Mark son of William Christian & Ann Kelly bapt Maughold Parish Church 9 Jan 1785?
Mark CHRISTIAN, 69, Pt y vullin, buried Maughold 21 Feb 1854.
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"William CHRISTIAN bt. 21 Jul 1751; d. 1834; m 10 Jun 1783 to Ann KELLY b. circa 1762; d, 1835"

William Christian & Ann Kelly married Maughold 10 Jun 1783. Ann was from Bride.

Assuming he wasn't under age which wasn't recorded at his marriage, there were two likely Maughold William Christians to be married at that date.
The one you have, who was recorded as William son of Ewan Christian (Mark) bapt 21 Jul 1751, and William son of William Christian & Elinor Christian (Balterson) bapt 20 Aug 1760.

In their Maughold book the Radcliffes give the lineage of the Ballaterson Christians, who were mainly William, John, and Edward.
I agree with your researcher who thought the "Mark" one was most likely - it wasn't a common name amongst the Maughold Christian families, which "William" was, so he was being identified as coming from a particular family.
There was another Christian at that time identified as descended from 'Mark' - Ewan's brother?
Jane daur of John Christian (Mark) bapt Parish Church 28 Apr 1751.

Maughold bapts of the children of William Christian & Ann Kelly:
CHRISTIAN Mark [son of] William Christian/ Ann Kelly [bapt] Parish Church 9 Jan 1785
CHRISTIAN Ewan William Christian/ Ann Kelly Parish Church 25 Mar 1787
CHRISTIAN Robert William Christian/ Ann Kelly Parish Church 13 Jun 1789 Private baptism
CHRISTIAN Joseph William Christian/ Ann Kelly Parish Church 6 Jan 1793
CHRISTIAN Thomas William Christian/ Ann Kelly Parish Church 7 Feb 1796
CHRISTIAN Jane William Christian/ Ann Kelly Parish Church 29 Nov 1801
?CHRISTIAN William William Christian/ Ann Kewley(sic?) Parish Church 14 Jul 1804 Private baptism

However your next generation: "Ewan CHRISTIAN bt. 16 Mar 1733; d. 1773; m Eleanor COWELL bt. 21 Oct 1729"
means that Ewan would have been only 17 when son William was born [Ewan son of John Christian (Clerk) & Joney bapt 16 Mar 1733/34], and wasn't son of Mark as on your list.

There was also a bapt for Ewan son of John Christian on 17 Mar 1714/15. So either could possibly fit, but both were sons of John.

The next generation: "Mark (John) CHRISTIAN bt. 27 Jul 1692" - this bapt is not in Maughold records, nor can I see it in extracted records on the IGI. The marriage 5 May 1717 was in Bride. You need to look at this on LDS film no.0106713 to see where he was from.

These are his probable children, but no sign of Ewan (unless in a will?):
CHRISTIAN William illeg son of Mark Christian/ [bapt] Parish Church 14 Nov 1710
CHRISTIAN Joney [daur of] Marke Christian/ [bapt] Parish Church 11 Mar 1717/18
CHRISTIAN John Mark Christian/ Parish Church 17 Jul 1720
CHRISTIAN Robert Mark Christian/ Parish Church 11 Feb 1725/26
CHRISTIAN Mark Mark Christian/ Joney Parish Church 30 May 1728
CHRISTIAN William Mark Christian/ Joney Parish Church 20 Nov 1730

Maughold MIs and burials have Judith (Joney) wife of Marck Christian buried 13 April 1765 (Jony CHRISTIAN als CROWE), and Mark Christian buried 18 September 1768, but no ages.
Will: CHRISTIAN, Mark 1768-3, Mau, Epis will LDS no. 0106390 - you can see this through a LDS Family History Centre (under 'Church of Jesus Christ of Latter-Day Saints' in the phone book). You could order the film (above) of the Bride register at the same time. Looking at the wills, earlier Mark Christians appear to have lived in Bride.

I'll be interested to see what the will reveals.

Sue