Clarifying Block/Charge in regard to RA

Player A1 drives baseline. Secondary defender B2 comes form the weakside, and establishes legal guarding position outside the restricted area. Player A1 initiates the contact, not trying to go around B2. B2 slides his right foot back as contact occurs, and his foot is then inside the RA.

My question: once a player established LGP outside the RA, can he move back his feet at all? If a player with LGP is moving backward as he is defending, moves his feet into the RA, and the offensive player initiates contact, what is the call?