The chest-jamming trick doesn't always work; I'd suggest just finding another glowstone. The places I always get my three: the first Mermaid, the lower chest in the two chests room, and the chest in the 3rd left tunnel (as you exit the mother crab room) in the moving cart/shark room.
Other than that, experiment further with the switches; what works for me is to click the right one, go and click the left one, then come back and click the right one again. That generally both opens the secret alcove and extends the bridge, though the bridge is superfluous (the gap can easily be jumped).
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