I dont think a violent eruption is needed. the '71 event created fractures that weakened the island. any seismic activity could be the proverbial straw. maybe even a few sticks of dynamite. As for chipping the island off slowly, even sending a tenth of that mass into the ocean at a time is still going to trigger a tsunami to the more local areas.
As for underwater walls, wouldnt that risk disrupting the corealis effect that we greatly rely on for weather patterns?