"We can rule out an emergent property of the three Hallows in combination. We can mostly rule out someone breaking the established rules of time travel (though I have to qualify that because I don't know how prophecy works)."

I'm not so sure. Harry has observed that magic should b e arbitraarily powerful, and was presumably invented to have the rules it does. Rot13 because of spoilers in a story EY referenced in author's notes, Ra: Jr nyfb xabj gung RL ernq dagz.bet/en orsber Whyl 2014, fb znlor ur jnf vafcverq ol gur angher bs zntvp va gung fgbel: 1) gung bevtvanyyl vg nafjrerq qverpg erdhrfgf, "Qb jung V zrna" nsgre fvzhyngvat shgher jbeyq fgngrf gb frr jung fngvfsvrf zl gehr, abg fgngrq, qrfverf, 2) gung guvf yrq gb qvfnfgre naq gur qrfgehpgvba bs gubhfnaqf bs vaunovgrq cynargf, 3) gung gur fheivibef perngrq neovgenel ehyrf gb erfgevpg hfr bs zntvp, naq 4) gung gur fheivibef gurzfryirf cerfreirq n zrnaf gb pvephzirag gubfr yvzvgngvbaf.