All stage 3Bs say when "Frodo's choice is revealed at this stage move onto stage 4B". Does that mean all other players remain in their own staging areas on their respective 3Bs till the end and one player has to face 4B alone? Because the only way they could join as far as I cna see would be by Frodo's choice shuffling back into the deck and getting discarded during their subsequent quest phases.

You stay separated. Luckily, you can contribute to the 'Frodo' player's quest by removing cards from his staging area, through the various stage 3 "when you quest successfully" effects. He's going to need your help, he gets a lot of cards!