Hi Lynne. You are correct. Your ancestor Margaret Gelling's maiden name should be Christian, not Kneal. The 1725 will summary for her in the Manx Notebook says her alias was Kneal but there is no mention of that in the will itself. Perhaps Frances could put a note in there that it's not correct.
There's a document in the MNB called "Kewleys of Ballafreer" that states Elizabeth Kewley als Gelling's mother was the daughter of William Christian of Balne Killey in Maughold. I know nothing more about him.
Going to the other one, the real Margaret Kneal matches the 1735 will. She was the widow of another John Gelling of Douglas with a 1724 Braddan will mentioning wife Margret Gelling alias Kneal, and also said he "cut off" son Charles Gelling other than 50s to be paid by his brothers John, Paul & William after decease of Margaret. The son John is the next 1735 will after Margaret's that says he died in Jamaica and had 3 brothers Charles, Paul & Wm. Wm is abroad, and a 1758 will for him says he left the isle 30 years ago, only bro Paul. Charles died 1743 and his wife was Alice Christian. The Alice in Margaret's 1735 will is referred to as "daughter in law", not daughter as you've said. I haven't figured out what happened to the last brother, Paul.
By the way, I think your ancestor Margaret Christian was the 3rd wife of John Gelling of Camlork. He was born abt 1634 (Marown?) eldest son of Paul Gelling Jnr (ca 1610-1663) of Camlork and Isabella Lewin. I think John's first wife was another Margaret Christian, d/o Ewan Christian of Maughold. The 1663 wills of both Paul Gelling & Ewan Christian help fill in the gaps. Wife #1 was buried 17 Apr 1657 "Marg't Gelling alias Christian". I believe wife #2 was Catharine Kelly; she was buried 5 May 1674 "Katherin Gelling als Kelley". I think Grandaughters Margaret Gelling and Jane Gelling mentioned in their grandfather Paul's 1663 are products of these two marriages. A Robert Gelling s/o Jo was buried a day after Catharine which is my hint of a connection. Possibly Joney & Mary from the registers belong here too; all children deceased by 1675.
I have more information about some of these people. Let me know where I can help further.
Best wishes, Eric.