“I’ve thought and thought about how the killer got in, and I’m sure he must have followed her inside when she came in that morning, because of Derrick Wilson leaving his desk and being in the bathroom. I thought Wilson ought to have been bloody sacked for it, actually. If you ask me, he was having a sneaky sleep in the back room. I don’t know how the killer would have known the key code, but I’m sure that’s when he must have got in.” (The Cuckoo’s Calling, by Robert Galbraith)
‘Would’ and ‘could’ are both used to indicate a possible thing, say OALD would #3 and could # 4. Then, can the ‘would’ in the novel be replaced with ‘could’ - ‘would' here doesn't seem to indicate irrealis but inference - with the meaning unchanged? Or would there be any semantic change if ‘could’ was used?