Yes, I think that is it.
The censuses fit for the two Marys for 1851, 1861 and 1871.
It would be interesting to find Lewis in 1851 but I can't. It might explain why there is no marriage to be found between him and Mary snr.
It is puzzling why those details were provided by her son. The naming of William Jolley as father can be understood - a mix up between Thomas and his father's name, however he was dead before she was born. In our theory, though, Mary Cunliffe was there through to adulthood.
As you say, it will have to suffice until something else turns up. I did spot a baptism for Mary, daughter of Mary - no father named so that may be the answer.
Regards
Heywood
