If Frazier doesn't get the job (through an employment agency) at the TSBD then Randle doesn't know about it. She can't tell Ruth Paine about any possible opening since she learned about it through Frazier (her brother).
If Randle forgets about the job then Ruth Paine doesn't know about it. Or if Frazier is fired, quits, finds a new job, et cetera.
If Roy Truly takes the call from Ruth Paine and says there are no job openings then Oswald doesn't get the job. If Oswald isn't interested, he doesn't get the job. If he has a poor interview he doesn't get the job. If he quits, gets fired (he had a history of this), then he has no job there.
All of these factors/possibilities (and more) are uncontrollable, are beyond the capabilities of humans to control. We all recognize it.
But in conspiracy world such matters are waved away. "The CIA", the "Deep State", the "them" and "they" are capable of anything. We continue to think we can reason with people who think like this. We can't.
Tracy Parnell goes over this in greater detail here:
https://wtracyparnell.blogspot.com/search/label/The%20Assassination%20and%20Mrs.%20Paine