Meaning of 万々が一


> しかし、 **万々が一** ポッターたちが関わっていたにせよ、あの連中が自分たちの近くにやってくるはずがない。  
>  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.




According to
[Weblio](https://www.weblio.jp/content/%E4%B8%87%E4%B8%87%E3%81%8C%E4%B8%80)
it's used to strengthen `万が一`, which is a set phrase meaning "if by any
chance; in the unlikely event that".




According to
[Weblio](https://www.weblio.jp/content/%E4%B8%87%E4%B8%87%E3%81%8C%E4%B8%80)
it's used to strengthen `万が一`, which is a set phrase meaning "if by any
chance; in the unlikely event that".

