The only value of trying to reattach the brick faces would be to maintain appearance, but as you say they’re going to be rendered and then painted there seems little point in attempting to do this, or trying to cut out whole failed bricks. If it were me I’d dig out the individual failed bricks with a hammer/chisel until you get to something solid, then patch the resulting holes with a small aggregate trowelable concrete mix (with a bit of SBR mixed in to improve the bond), then render/paint as you were intending.