I think the 4E term for being unable to move or attack is "helpless". Letting a single maneuver or power do that would be pretty game-breaking. I'd say that if you reduce someone to 0 HP or below, at your option you can define that as grabbing, disarming if needed, and pinning them rather than unconsciousness or death.
The "restrained" condition (immobilized plus grant CA, take -2 to attacks, and can't be forced to move) is a reasonable result IMO of a second successful grab on an already grabbed target...with the exception that the grabber can still move the target per the grab rules. There's no rule I'm aware of, though, for "restraining"; I'm home-cooking here.