Hello Anne,
Did the professional researcher not cite his/her sources for the information? Or his/her reasoning and proof for the information which was provided to you?
The scenario you draw sounds reasonable to me (I wonder why you doubt it) - and the information about the births and deaths of Alexander and Jane must have come from
somewhere! I would try to find out where it came from - the only source would seem to be parish registers (given that the stated births, marriage, and deaths of Alexander and Jane were well before the time of statutory registration though Jane is supposed to have still been alive at the time of the 1841 census). I certainly would not be at all worried about a 12 year age difference between Alexander and Jane (which you seem to be if I understand you correctly).
If I were you, and if you have not already done so, I would be working steadily and carefully backwards from the death of Margaret (NICOLL) KENNEDY. I would start by searching on the pay-per-view site ScotlandsPeople at:
http://www.scotlandspeople.gov.ukfor Margaret in the 1881 and 1871 census which are available there. These should give her place of birth, and also possibly a more accurate age than is on her death certificate (though a discrepancy of 5 years compared with Margaret chr 1802 is not unusual; the older the person, the less reliable the age is likely to be).
The marriage of David KENNEDY and Margaret NICOLL seems to be in the IGI - you could order in the film of the parish register to your local LDS Family History Centre in case it contains more information (though often the parish registers don't have any information other than that in the IGI).
David KENNEDY mar MARGARET NICOLL, 1 Apr 1834, Dundee, Angus.
Most of the 1841 census for Angus is on FreeCEN at:
http://freecen.rootsweb.com/though I can't seem to find David and Margaret.
There are a couple of possibilities for Jane/Jean NICOL/NICOLL (your information says she died in 1842).
Good luck,
JAP