However, ??? even if it did concern the Potters, there was no reason to expect that that lot would turn up in their own neighbourhood.
I haven't the faintest idea what 万々が一 means in this sentence. I understand that 万々 means 'very much', but why would an adverb take が (besides which, I can't see any way to fit 'very much' into the sentence)? There also appears to be two subjects in 万々が一ポッターたちが関わっていた.
I don't know if 一 is part of the same problem or whether it somehow prefixes ポッターたち。Either way, I don't understand this part either.