Some outside observers' current best guess about mitigating this danger is to have the returning boost stage initially aim for water just off-shore, which leaves time to destroy it in the air if it winds up off-course. If things look good, they can then use the landing burn to divert from just off-shore to just on-shore.
FWIW, they haven't announced anything specific about range safety management, but the "Grasshopper" landing test rig was seen rehearsing such a horizontal diversion on one of its last flights.
(As to the barge, its being mobile is a feature, not a bug: they could tow it to whatever spot at sea needs the least fuel to get to after stage separation. The problems are that the barge itself may not stay level during landing, and that the rocket exhaust from even a single throttled-down engine is still likely to burn a hole in a barge.)
FWIW, they haven't announced anything specific about range safety management, but the "Grasshopper" landing test rig was seen rehearsing such a horizontal diversion on one of its last flights.
(As to the barge, its being mobile is a feature, not a bug: they could tow it to whatever spot at sea needs the least fuel to get to after stage separation. The problems are that the barge itself may not stay level during landing, and that the rocket exhaust from even a single throttled-down engine is still likely to burn a hole in a barge.)