Why did OSI assign their best agent to protect the Venture family in the first place? What was his actual mission?
I think he was assigned as the Venture bodyguard as punishment for investigating the Guild of Calamitous intent. (See the Invisible Hand of Fate episode in Season 3)
Rusty Venture is a failure as an adult and living in his father's shadow; it's considered a punishment to be assigned to him. Sgt. Hatred suffers a similar fate when he replaces Brock.
Brock's other mission (or real mission) is to kill Rusty if he ever tries to activate the mysterious orb as revealed in Operation: Rusty's Blanket
His real job was to protect the orb, not the ventures. In fact he was told to kill Rusty if he ever took the orb, but then he found out the orb was broken and decided against it.