Hi
First, have checked original indexes and there is no Mary Emma Richards (or even plain Mary or Emma Mary) in the Dec.qtr. 1869 or March qtr. 1870 at Helston so either her birth wasn't registered (maybe born around time father died and it got overlooked??) or there was an error and Helston District didn't submit it to the General Record Office. You could contact the local office giving them Mary Emma's birthdate and ask them to check their records to see if a mistake was made but if they haven't got a record of it then there is no way you will be able to get a copy of her birth certificate as the GRO have no note of it. I do not have any other details regarding the possible marriage other than that already mentioned.
The census details are:
1881 - Penhale Downs, Breage, Cornwall
Elenor Richards Head Widow 45 Housekeeper, Miners W. b. Sithney, Cornwall
William Richards Son 20 Farmers Servant b. Sithney
James Richards Son 16 ditto b. Breage, Cornwall
Susan Richards Dau. 14 Domestic Servant ditto
Marey E. Richards Dau. 11 Scholar ditto
Edith Richards Dau. 7 ditto ditto
William Wills Boarder 48 Tin Miner b. Stithney
Ref. RG11 - 2326 - 46 - 11
(in 1891 Mary E. is working as a servant in Helston, Eleanor apparently married the 1881 Boarder William Wills - Jun. qtr. 1882 Helston - and although shown as married in 1891 no sign of William Wills - Eleanor's occupation is 'Maintained by sons' and with her are Grace Richards Step-dau. 23 'Husband Abroad' - think this should maybe be dau. in law) and Edith Richards 17 Dau. - Ref. is RG12 - 1842 - 32 - 7)
1871 Census - Merther Sithney, Sithney, Cornwall
Eleanor Richards W 31 Charwoman b. Sithney
William Richards Son 9 "
James Richards Son 6 "
Susan Richards Dau. 4 "
Mary E. Richards Dau. 1 "
Ref. RG10 - 2302 - 64 - 4
1861 Census - Sithney, Cornwall
William Richards Head 36 Tin Miner b. Breage, Cornwall
Ellen Richards Wife 26 b. Sithney, Cornwall
Willm. Richards Son 4 mos. ditto
Ref. RG9 - 1571 - 37 - 20
Elllen/Eleanor somewhat inconsistent with her age!!
Baby William in 1861 was, as mentioned before, bp.31/1/1861 Sithney, son of William and Eleanora.
Hope this helps.
Annette