Rosie, it certainly is a sad outcome to what seemed a straightforward request where you might have expected them just to be hiding under a poorly transcribed name - not (as now seems likely) that all except little Sarah had passed on, and Sarah was in the workhouse.
If the dc turns out to be your Samuel, and if the informant is his wife, then that would at least rule out the 52yo Mary Ann HARRIS death Dec qtr 1871 in St Saviour, and leave the more likely (because of the location) 61yo 1880 death in Lambeth.
I had wondered whether the reason 28yo Mary Ann TEBBS & baby Mary Ann TEBBS (if they are yours) were registered in St Saviour might have been that they were in a hospital there and a hospital person might have been the informant of the deaths; the birth is also there in St Saviour - Mary Ann A TEBBS, Dec qtr 1872 St Saviour.
Ah well, I guess you now have the name and occupation of Frederick's father from the marriage certificate and thus some more investigations to do along that line - I hope they are more cheerful.
JAP