Well, Tom could have been approached by Roth just as Fredo was approached by Roth. In neither case was it necessary for Roth to know what they knew. he was trying to find knowlege of Mike's plans.

Your point about Rocco possibly becoming the new Pentangeli if he were captured is a good one. But I doubt that Mike believed that the Feds would buy any kind of story about Rocco being a security guy. Given Mike's high profile as evidenced by the Senate hearing he was subpoened to, the Feds would have the family org chart mapped out with Rocco's picture prominently posted. I still say it was an FFC theatrical device.