I have your pm with Hannah Fish's details on the 1871 census, this matches up to young Hannah in the 1841 census, except that you quote she is married, but in my opinion it reads unmarried, which makes more sense otherwise we'd be trying to figure out whether she was a daughter in law instead. I'm still trying to figure out where she and Mary Ann are in 1851 and 1861, if with relatives they might have been incorrectly listed under a different name, but .......... try this for size!
Mary Ann Fish married in December qtr 1870 Blackburn, 8e 566, to either George Clarkson Gray or James Livesey.
Could this be a relative of the witness to the marriage of John and Melinda, although likely it would be a descendant since there is quite a number of years in between, it just seemed coincidental.
Having said that I'm not able to find a newly married couple with those names on the 1871 census, but of course it could be difficult if James had a previous marriage and they had children with them not matching up to the marriage date. This is a possibility since Mary Ann would be over 30 if this was her. There would be no way of confirming this except by the marriage cert to see if he was a widower.
Anyway, it's just a thought.