re Ann vs Mary on 1881 - who knows ! I've seen the strangest things on a return. The enumerator may have misunderstood, may have copied his schedule incorrectly into the formal return (the image you see on Ancestry) and and and ...

We dont know the actual date of the marriage 1Q 1881 - I'm just assuming it had to be before the census (Census was taken on the night of 3 April 1881) .... and people do get enumerated in 2 places at once very occasionally ...
All the other points about missing daughters from 91 to 01 can probably be explained ...