I can’t add any more to the regulatory side of things, but is it not the case that if we assume a uniform distribution of cloud/mist/fog giving a met vis of 600m, we could reasonably expect an RVR measurement to give more than than 600m?
Of course it doesn’t always work like that in the real world - especially in the ranges of visibility we’re talking about here. It’s not unforeseeable that a rogue fog bank could envelope the TDZ RVR on such a day. But the chances of that happening on a day when one didn’t have the ability to carry extra fuel for another div and or holding without - say - offloading freight are pretty low, and one would still have their original alternate which must have met the more stringent requirements - from an EASA OPS pint of view. In that case - if you gotta go (to the alternate) then you gotta go...