The unsigned and undated Note to Marina that doesn't even mention Walker is worthless as evidence
The confession to Marina isn't worthless but it should not be taken to heart in the absence of corroborating evidence. Like for example, did she tell anyone about it before her husband was killed? Did she keep a diary? Her word alone isn't enough given her credibility problems.
The other problem I have is no one seems to be able to explain how Oswald traveled across town and back with a rifle without being noticed. There's also conflicting stories between Marina and George DM about whether he came home with the rifle the same night or if he buried it near Walker's house and picked it up a few days later.
The note was found by Marina on the night of the Walker attempt. It explains to her what to do in the event that he is arrested or killed. Now what exactly would Oswald have been up to that might have led him to be arrested or killed that night? It was wasn't his typing class. The note is highly incriminating in that context. But if you are entertaining the idea that it was faked to link Oswald to the Walker shooting, then why wouldn't the conspirators have done exactly what you suggest and make it more explicit: "Dear Marina, I'm off to kill the fascist Gen. Walker. Don't wait up. Love, Lee Harvey Oswald." This one is a slam dunk. Oswald confessed, he wrote a note explaining to Marina what she do if he were arrested or killed, he had recon materials of Walker's home, no alibi for that night, and no other suspect has ever been identified in over fifty years. It's a compelling case. The baseless suggestion that Marina is lying for some unknown reason, won't set the record straight, the note was forged or Oswald inexplicably expected to die or be arrested for some unknown reason is just going down the rabbit hole.