Ruby stalking Oswald that weekend is proof enough for me he was chosen to silence him. The last thing DPD wanted was to kill their suspect and had no hand in any plot. Ruby would never break the Mafia's code of silence Omertà.
The DPD had several golden opportunities to kill Oswald with complete justification had they wanted to silence him. But you think the plan was to take Oswald into custody, allow him to speak to the world press, allow him to speak to members of his own family, then recruit an unstable Ruby to kill Oswald on national TV and then remain silent for the rest of his life in jail. And Ruby sleeps in that day, takes his dog with him to the hit, but only after stopping to send a meaningless money order which makes him nearly miss the transfer. Wow.