Well that is what conjecture gets you......hints in the right direction but not always the correct answer

Basically you'll just need to buy a view of the original 1851 census on SP and it's fine to go ahead and search on Annabella to get to the right page...but you'll quickly see that it's not her household that interests you. Matter of fact Ancestry has muffed up the household entirely as 60year old Christina doesn't even belong there!!!!! Shes in the household just above......but keep looking up and just above that is your John age 20 and wife Christina McTagg age 19!!
As for the parents of Christina per IGI being Alexander McTaggart and Christian Shaw. I guess you already know that is not what her DC states. The IGI is a match on age and the DC info was provided by a son. Until you prove otherwise you should stick with the son but believe me they have been known to get their grannies way wrong before!!!
There is only one Chris* McTag* on the 1841 that fits and she's an orphan....
Name: Christine McTaggart
Age: 12
Estimated Birth Year: abt 1829
Gender: Female
Where born: Argyll, Scotland
Civil parish: Campbeltown
County: Argyll
Address: Saddel Street
Occupation: Orphan
Parish Number: 507
Household Members: Name Age
Janes McGregor 68
Margaret McGregor 58
Christine McTaggart 12
I'm working on a theory at present that would make this household of folks possible older siblings or half siblings to Christina as I have the death of a Charles in 1879 age 64 at the poorhouse and his parents are given as Charles and Christina mn Stalker.
Civil parish: Campbeltown
County: Argyll
Address: Georges St
Occupation: Labourer
Parish Number: 507
Neighbors: View others on page
Household Members: Name Age
Neil McTaggart 29
Charles McTaggart 26
Mary McTaggart 19
Margt McTaggart 16
Not making much headway on it yet though.....
You might consider a trip to Edinburgh to try out the new research Centre there so you could maximise your access to the records. It's starting to look like the whole of Cambeltown has the potential to be relatives
Best wishes
Jean