Disprove? Go back and read what is written under the greentext assumptions.
I will err on the side of you not being an shill and just an anon defending your submission passionately.
Your math is not relevant because you are solving a problem that is based on a model which does not accurately represent what is currently known about the island. You have proven there may be a tunnel, which is essentially already known. Your final answer being a fixed volume then solved for length does not add any further information to our understanding of the tunnel network.
Continuing to base future ideas/hypothesis/etc off of a flawed model will only steer us in the wrong direction. Not that the effort is not appreciated, Anon.