You actually don't even need the mimics' help at all for this one, it's all about trapdoor efficiency.
×In my solution I pushed one mirror into the water using the farthest reach I could, then brought two mirrors simultaneously to the corner and had just barely enough room to shove each across.
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×That is, I faced South and stepped Southeast to push one mirror into the southmost row...
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× and brought two mirrors to the southeast corner and the square above the southeast corner, respectively.