Any employee who has a key role & is listed as a witness would still be expected to testify, irrespective of whether they continue to be employed or not. I had a civil case a few years ago I took over (IANAL) & the person who did the investigation (who left) was the one called to give evidence as he'd compiled the investigation. It was his statement in the bundle, so he was the one who had to have his evidence questioned.
With KS, I don't know if NHS Fife would fund, for her, her own legal representation (as NC intends to ask for her to be added as a respondent) if she left, I don't know how that works & likely depends on the terms of the contract & ins/legal cover that might specify. I don't think her attendance can be compelled, but massive inferences on a failure to attend & answer Qs on her actions/statements in the aftermath of what happened.
I had a separate case involved in a police prosecution where I was summoned as a witness (my statement was key evidence, so I was told) and the police sent officers to my work to find me & serve a very short notice summons (they apparently couldn't contact me - they had my email 😒) & a failure to turn up in a criminal prosecution I think has more serious implications. Civil court, which is what SP's case is, there's not a lot can be done with failure to attend.
I'm not that knowledgeable on the legal side of things but I've just been involved in litigated cases a long time so seen a few different things linked to trials etc. as a result. It's still guess work re SP's case & the other witnesses still to be questioned.