Oswald didn't "get a job on the parade route." In fact, that was impossible under the known timeline. There was no parade route when Oswald obtained his job. Oswald did not "strut his stuff plotting to murder the President." He didn't even know that JFK was coming to Dallas until just a couple of days before the event. He didn't make any threats or discuss his intention to assassinate JFK with any other person. He just did it. There was nothing to uncover to suggest he was intending to assassinate JFK. The FBI knew only that he had pro-Communist political views. What difference would it make to remove Oswald from the watch list if the FBI and CIA were in on the plot? That doesn't make any sense. What were they going to do? Arrest the guy they planned to frame for the crime?
Oswald's "history" comes, in part, from people like Marina, Kerry Thornley, Michael Paine, Robert Oswald, Soviet/KGB agents and others. If someone thinks this "history" was concocted, created by "the government" - that those people read from scripts (even the KGB agents?) to frame Oswald - then what can you do to counter this? Whatever evidence you produce will be dismissed as part of the fake history made by "the government" in their efforts to kill JFK and frame Oswald.
Item: We have the account by the KGB/Embassy people of a irrational Oswald who feels persecuted by the US. This is not a well man, this is not someone, for me, the CIA or anyone could rely upon. He's unstable. But to the conspiracy believers this was really not Oswald; it was an impostor. So where do we take this?
As to the job: As you pointed out, Ruth Paine, on Marina's request after they learned from a friend (Frazier's sister) that a job might be available, got him an interview. That was it. After that it was out of her hands. Oswald showed up and applied for it. He made the impression. He took the job. Truly hired him. Oswald performed the job. He showed up for work. Who was able to control all of this? We see chance, fate, terrible luck taking place. Conspiracy believers see invisible or not so invisible (Ruth Paine) hands directing all of this. It's crazy.
If you ask 15 different conspiracy believers/hobbyists to give your their story as to what happened, their theory, their explanation, you get 15 different explanations. They don't agree on who killed JFK. Was it the CIA? A rogue element? The Pentagon? FBI? Rich Texas oilmen? The mob? Anti-Castro Cubans? Who? And what about Oswald? Was he a willing participant who was used? Or an innocent bystander? Or something in between? There's no agreement on any of this. They are all over the conspiracy map with their explanations.
It's like some weird game, "JFK: The Assassination" where the participants try to score the most points by coming up with the most novel and innovative claim. If it's just a hobby, then fine (I guess). But some of these people are very serious. That's no so fine.