If they were going to kick him out on that basis alone, then they would have done it ages ago. The fact that PM maintained a friendship with JE after his conviction has been in the public domain for ages. It isn't news, and we have known for a while that this fact was known to KS at the time the appointment was made.
Rightly or wrongly, KS remained in post despite all of that, having argued that he had not realised the depth or extent of PM's friendship with JE.
What will hopefully emerge from the documentation when it's published is a much greater degree of clarity about what exactly KS did actually know and what he didn't know; the extent to which any warnings may or may not have been given and whether or not these were acted upon or ignored; and indeed the extent to which it might be deemed that any "due diligence" that was done was sufficient and appropriate for the seniority and risk profile associated with the Ambassador role.
I think Starmer's survival as PM, at least in the short term, is probably dependent on the detail that emerges from those documents. If it turns out that he ignored clear warnings and/or that he knew much more than he has previously indicated, then he will have to go. If the evidence largely backs up his account to date and PM was given clearance after the vetting process, then he might cling on at least until the May elections or perhaps even longer.
Regardless of how things pan out for KS, I think this whole sorry saga highlights serious issues with how our systems are operating. If KS didn't know more about PM's previous behaviour, then it sounds like the vetting process is sorely inadequate - surely our intelligence services should have been able to find out enough to have been able to warn KS that PM was a significant risk? And if KS did know more details, was warned against appointing PM and then chose to appoint him anyway, why on earth are we allowing one individual to exercise so much power without having any proper checks and balances in place? Either way, the systems and processes don't appear to be serving us well.