What I am saying, badly, perhaps, is that Oliva, and Nigel,, by being at the location where the murder is stated by the prosecution to have happened, and being there when the Crime squad were called in to that location, * Allisons home* , being there alone with Gerard before the police first responded, they are part of the body of the crime. Both will have to testify, on Gerards behalf, as to when they got to that house. Why they went there. Who called them. When. What the person said when they were summoned. Its Gerards story.
Neither of the lived there, it isnt their abode, they had to come from somewhere else to be there, by some form of transport, on public roads, etc.. It isnt Gerards story that Nige and Olivia stayed the night or the week, or dropped from the sky, or materialised out of the ground. They are an intrinsic part of Gerards narrative as to his innocence. It could be said they are the only intrinsic part upon which his innocent claim hangs.
Its on these grounds that I believe they will take the stand as defence witnesses, but that leaves both of them open as it does to every other defence witness of examination by the prosecutor. It's difficult for me to see them not testifying. Its a huge huge hole in Gerards timeline without the both of them, they are crucial to it.