So what you are saying is that a case based on circumstantial evidence can never be made out.
I didn't say never. But not when there is just a small amount of circumstantial evidence that is all weak and tainted, no.
In this case, the circumstantial evidence against Oswald, which includes what he did after the assassination, is overwhelming.
All I can say is, you're easily overwhelmed.
Even without the evidence that Oswald attempted to kill General Walker (which would almost certainly be admitted as similar-fact evidence)
Let's talk about that evidence. You have a secondhand report of something he supposedly told Marina, some photos of Walker's house, and an unsigned undated note in Russian that doesn't even mention General Walker. What else?
the evidence leads to only one reasonable conclusion.
I completely agree. The one reasonable conclusion is "undetermined".