Ahh, a complicated issue to untangle
To be honest I had a niggling doubt in my mind when I saw the Ancestry trees because there was a fairly large discrepancy with William's age. His age at death is 89 (I checked the index) which gives him a DOB of about 1831. This ties-in with his age given in 1911 (80) Of course both of these ages would have been given by his son, so subject to being incorrect.
Going through the census,
1851 - born Maybole, Ayr age 10 (1841)
1871 - born Crosshill, Scotland age 32 (1839)
1881 - born Argyll, age 41 (1840)
1891 - born Scotland, age 55 (1836)
1901 - born Maybole, age 69 (1832)
1911 - born Dundee, age 80 (1831)
Yes, I found that BRIDE marriage (after much looking) in 1870 but the reference to Ann in 1911 is definately an error from the transcriber as, if you look closely, it says, 'McBride'.
To prove his date of birth I would suggest you pop over to the Scotland board (Ayrshire) & see if anyone can find his baptism (civil reg. in Scotland started much later) His burial might not be of any help to you as, 1. his son would have dealt with it & probably didn't put a DOB on the headstone & 2. if he did, it would probably be the same as the 1911 census.
Have you contacted anyone on Ancestry regarding this Surrey William?
Records for Russell school might be at Croydon Local Studies. I've tried A2A
http://www.nationalarchives.gov.uk/a2a/ but can't find them at the moment
Jane