I'm not sure what the 'case files' are, workhouse admissions and discharges generally give name, age, address before admission and information on where and why they were sent out, but that's assuming 1918 would be the same as the ones I've looked at from the 1880s/turn of the century. Sometimes there are extra notes given.
The information you'd be most interested in I would suppose would be Ethel's age. In addition you could find out how long she'd been in the workhouse, how long she stayed, if she went back to Melville Road or on to somewhere else - if Desmond left with her or if he was given up then - etc.
http://www.movinghere.org.uk/browse/v.htm - if you click on 'whitechapel workhouse' you can see examples of an admission/discharge register and creed register. You'd have to ask the LMA what the Case Registers hold, probably they contain other information relating to some of the inmates.
As it was only in 1918 that women over 30 gained the right to vote (and women over 21 in 1928), the electoral registers wouldn't show her at Marville Road (would show if there was a Jarvis around there, though, or a male Salter who might be a family member).
You could also possibly trace forward to see if an Ethel Salter married in the Chelsea/Fulham area soon after 1918, but it would be hard to prove it was the same Ethel.
