DL and her agenda are IMO quite a big part of why SJL's life developed as it did and why the search for her failed. DL was out to protect SJL's posthumous reputation, and the misapprehension she held, carefully-curated by SJL, of how she had lived her life. Nothing else mattered more. DL was not about finding out what happened to SJL if this involved accepting even that she wasn't a 25-year-old virgin, never mind that she had slept with dozens of men. DL wanted an account of SJL's fate that showed her to be a blameless innocent - which by pretty well everyone's standards she was, but not by her mothers: those of 1950s East Sheen. DL wanted this more than she wanted SJL found; more than she wanted the truth.
The instant attempt to shut down an innocent conversation with the flat mate is evidence of this. She wouldn't tolerate even a hint that SJL might actually have been sleeping with a man. That would mean, in the parlance of the day, that far from having a lodger, SJL had "shacked up with" a bloke and was "living in sin". Yuck. Won't have that said.
DL had previously attempted to police her daughter's love life and deliberately interfered in it without hesitation. This attempt at control led SJL to seek work and any other opportunities that would allow to her to move out of home ASAP. She did and she heavily edited her accounts to DL of what was happening in her life.
This may have doomed her. Nobody but SJL seems to have known what went on between her and PSS, for example. If she had told her parents about things in her life that were worrying her, someone worldlier might have warned her. Her dad might have told whomever this bloke was that was "hassling" her to leave his daughter alone. But she did not have those conversations with her parents, and as the eldest sibling, nor had she any big brother or sister to confide in either.
So SJL's backstory is acutely relevant, but just not in the way usually assumed.
This is IMO why DL was so keen on JC as a patsy. His history of being a smooth and persuasive liar meant that a trusting innocent was duped. He was the perfect killer SJL could have done nothing to save herself from, and who stalked her after he looked at her in a shop window. She died because she was pretty, not because she was a hussy. For DL, that account is much preferable to any in which her associates and choice or number of bedmates is a factor. So that's the account that gets pushed, and as a result of this misconceived reputation management exercise, someone gets away with it.