My final solution, already submitted as a review to fanfiction.net:
My three ideas for persuading Voldemort to let Harry out of the box, or at least for buying Harry enough time to escape by other means, using all true statements in pseudo-Parseltongue:
1:
Harry: “There iss already in motion a power which will desstroy the world if it iss not sstopped. [entropy/heat death] Even if you kill me now, or try to limit my influence on the world, the world will sstill be desstroyed by thiss force.
I am sstill bound by the vowss I have jusst sworn to you, and I will not act in any way I believe will causse a greater desstruction, or a higher chance of desstruction. I alsso genuinely intend to sstop the desstruction of the world, and I genuinely believe I can accomplissh thiss [given enough time and resources], if you let me.”
2:
Harry: “I am nearly certain that the prophecy you are trying to avert *will* be fulfilled, in one way or another. I have genuine reasson [Comed-Tea experiments; can explain if Voldemort asks] to believe that magic hass accesss to the whole of time, and already knowss how the future will unfold. If I am correct in believing thiss, there iss no way you could sstop the fulfillment of the prophecy, but your actionss now may be able to influence *how* it will be fulfilled.
While I cannot be ssure without knowing the true wording of the prophecy, there are likely many outcomess which would fulfill the literal requirementss of the prophecy while leaving the world in a sstate that you, I, and the girl-child could all agree iss not ssignificantly worsse than and perhapss even better than itss current sstate. If you kill me, or try to limit my influence on the world, you will esssentially be leaving how the prophecy will be fulfilled up to chance, but if you let me work with you, we can greatly increasse the chancess of a possitive fulfillment. I have already been bound by your vowss, and I tell you all thiss in good faith, without any intent to trick you.”
3: [uses material implication to mislead as suggested by dxu on LessWrong]
Harry: “If you or your minionss kill me or girl-child, harm uss, resstrain uss, incapacitate uss, blackmail uss, or otherwisse annoy uss anymore than you already have, the world will be desstroyed. [by eventual heat death, which it would have been anyway] You will command your minionss to sstand down, you will awaken girl-child and let her walk to me unhindered, you will lay my unaltered posssessionss at my feet, you and your minionss will leave thiss place without caussing harm to me or girl-child or arranging for harm to come to uss after you have left. If you do not do ass I have told you, the world will end.”
My final solution, already submitted as a review to fanfiction.net: