Go with your
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×first
plan.
The problem is: either the briar cuts itself off before it kills the gel mother, or it kills Val, right? Both of those things happen because
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×the briar toggles pressure plates.
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×So you need to get the plates in the desired configuration and not let the briar toggle them after that.
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×You've probably already proven to yourself that Val can't do this by himself.
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×Why not leave something on the appropriate pressure plates so they stay pushed down the whole time?
Please ask if you need more