It's confusing isn't it?
I do wonder now if the Grace in 1871 with Harry is like you say not Grace at all, but Harriett, especially if Grace is showing in Cornwall in 1871.
I've found Grace back in cornwall in 1881, 1891 and 1901 and her age makes her a lot older. But still odd that the births of the children are mixed up between both mother's.
As far as I can tell all children were registered in Bermondsey/St Olave Southwark, none in Cornwall.
There definitely seems to be some odd stuff going on, as Harriett, majority of the time gives her place as birth as Surrey on the census, except for once when she in the workhouse in 1900s she gives her place of birth as Cornwall. It definitely appears she was born 1842/1843.
So Harry married before Grace? I didn't notice the widow part. He definitely doesn't appear to of married Harriett.
Then there is all the confusing stuff with daughter Catherine too and being with Benjamin Brett, but appearing to not actually marry him, then reverting back to Fisher and ended up in the workhouse too with her parents, and continuing to have children even though she is destitute, before finally getting out and marrying Charles Powell.
I was convinced Martha's husband Daniel Brett was the brother of Benjamin, but he doesn't appear to be.
I will look at some of the other stuff you have posted when kids are in bed later.
There is the Grace Mitchell to look into too, who Grace Fisher (nee Swine) is with in 1861 and is shown as her grandmother.
