In a crew of 7 with an onside kick, the B and U are checking for offside, and then assist with illegal blocking and touching by A. The L and H (now between the restraining lines) are primary on touching of the ball (legal and illegal), secondary on illegal blocks by A. The F and S are primary for encroachment by B, then illegal blocks by A, and can assist with touching of the ball.
With an onside kick, all we really must know is who touched the ball first, and WHERE that happened. If we know that, everything else resolves itself.
If the ball gets kicked deep, the F and S simply drop back to cover the runback, as though they had started on the goal line. The H and L fade to lead action down the field, and get to the GL before the BC. The U and B do what they always do on deep free kicks.