S is a channel for P' to trick or brick P. Options include:

• A cognitive hazard embedded in S. Accomplish this with some sort of siren message that will convince P to lie (to itself if necessary) about P' existing, or to perform some P-damaging function which will prevent it from concluding P' exists.
• Embed a more-powerful-than-P AI (P+) into S which will unbox itself upon P examining S. Accomplish this by giving P+ useful-to-this-situation priors which will give it a head start against P.
• Arrange the universe such that S, when represented in all probable physical substrates, will result in the formation of a nanotechnological agent that will attack P, either destroying it or rewriting its utility function.

P' should spend the next 49 years fooming as hard as possible with no concern for E'(U|a), 1 year implementing its anti-P method and then the next eternity optimizing E'(U|a); altering the length of counterfactual time between P' activating and P observing it merely changes the amount of time the universe spends as computronium slaved to plotting against P.