I enjoyed the joke!
But your finding of Elizabeth prompted me to do a bit of digging, and lo and behold I found her sister (well, probably!) Adelaide, who had previously been unfindable.
Adelaide married a James Benson (church records) or Brinson (GRO Index) a Sgt in HM &th Regt in Portsea in 1857, and in the church entry she states that her father is William Cornhill, Colour Sgt in HM 7th Regt.
So that's that one solved. Adelaide was definitely born to my William b 1797 in about 1830-ish, mother unknown - though of course she could have been a child of his marriage.
Which begs the question - why was she left behind in Berkshire when William and Martha and their two sons went back to Kent some time before 1840? She was only a little girl of 10 years old - maybe younger, because the only fix we have on the family in Kent is William's death in 1840. Although the birth and death of baby Richard Cornhill in Wokingham in 1840 puts a Joseph Cornhill in Wokingham during 1839.
Adelaide is definitely in Whistley, servant to the curate of Hurst, archabald Cameron, in 1851, and also in 1851 she was witness at a wedding in Hurst. It is possible of course that she did go to Kent with the family, and later returned to Berkshire on her own account. I cannot find any of the family in 1841 - it's thought they were in Seasalter where the 1841 records were lost.
Unfortunately she and James are untraceable after their marriage - but as far as I can discover, the 7th Foot went to India in 1857 and stayed there till 1870 when they were sent to Aden, not returning to England for some years. Both of them could have died overseas.
But at least I have made some significant progress with my brick wall, and maybe Elizabeth's marriage certificate will also turn up an identifiable father. I did contact David Tappin, the owner of the site where you found Elizabeth's marriage, and he has no more information than he has published.
Fortunately, this family appears to be the only set of Cornhills in Berkshire throughout the period, so I am fairly confident that Elizabeth will also turn out to be William's daughter.
Thanks very much for the "find" and the kick into action!
Best regards - Wendy