One more point: This whole question only arises according to the opinion in the gemara that Eruv Tavshilin was instituted for Kevod Shabbos. But according to the opinion that it was instituted for Kevod Yomtov the whole question doesn’t arise in the first place. According to that opinion we are not really relying on hypothetical guests; that story is just a polite fiction in order to spare Yomtov’s feelings. Everyone understands that we’re really cooking for Shabbos, but for Yomtov’s sake we pretend we’re cooking for guests that might show up, and that we really intend to subsist on Shabbos on just the egg and the matzoh that we put aside on Erev Yomtov. Since this is all just a pretense, it doesn’t matter how thin it is.
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