I read it as If the spells' effects had been reversed (but Dumbledore apparently thinks they were not), all that would have happened is/was a kind of reverse echo and a shadow of the living Cedric would have emerged .
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contrapositionWhat meaning is intended by the two 'would have past participle'?They are counterfactual statements, like almost all "would have" + past participle constructions. They are imagined results of an imagined situation. (So yes, there is a conditional component.)