First of all, whilst witnesses were sometimes parents, more often than not they were not and could be other family members, or unrelated at all.
Since all but the last of their children were born in Wyke Regis, they appear to have married there, surely Mary Harris is likely to have been one of these two:
Mary Harris bp.9/4/1795 Wyke Regis, dau. of Joseph and Elizabeth
or
Mary Harris bp.26/10/1793 Wyke Regis, dau. of Uriah and Ann
From looking at the baptisms it would see Thomas and Mary had 11 children baptised at Wyke Regis
1815 to 1835 - besides ones shown on census they include a Lydia Adams 15/5/1831, Elizabeth 25/12/1815, another Margaret 29/11/1829, Louisa 30/6/1821, William Henry 17/1/1819. Presumably the latter 3 above must have died since they went on to have further children with these names.
I would say you have the right marriage and that as a William Russell is one of the 2 witnesses there is probably some family connection somewhere and could be why family believed Mary's maiden name was Russell. Probably just a red herring, but a William Russell actually married a Mary Williams (name of other witness) at Shaftesbury 24/10/1817!!
Annette