Rearranging this a bit to group similar points into connected quotes.
That seems a rather round-about approach. Just replace "a period" with "at least 1 hour".
Just replace "a period" with "at least 1 hour" and you are there.
"If the rest is interrupted by at least 1 hour of strenuous activity—walking, fighting, casting spells, or similar adventuring activity—the characters must begin the rest again to gain any benefit from it."
This wording would seem to suggest that it must be at least 1 hour of walking, or at least one hour of fighting, or at least one hour of casting spells, etc. whereas the current reading suggests a period of any combination of the listed activities totaling an hour.
It's very problematic, from a system design point of view. Seeing as 1 hour of fighting never happens, you are left with -
A = period of strenuous activity (1 hour of (walking, null-condition, casting spells, similar adventuring activity))
- which produces the knock-on problem of what is going to be similar to an absurd or null-condition!?
(...)
Are you saying that 600 contiguous rounds of fighting over the span of a single hour is plausible, rather than an absurd or null condition? There seemed to me quite solid consensus on the absurdity of that: are you reverting that argument?
Fighting is not a null condition. Any amount of fighting contributes to a period of strenuous activity, it doesn’t have to be entirely any one item on the list. It
can be. It probably
won’t be an hour of only fighting. Frankly, it probably won’t be an hour of any one thing alone, because the players aren’t idiots and will stop doing the strenuous thing at 59 minutes at the most. But if your wizard spends 59 miniutes casting spells, she’d better hope no monsters attack, or thieves steal any of the party’s stuff, or anything else that might cause her to need to fight, walk, or do something similarly strenuous for a minute or more.
It seems strange to me here to omit the "or similar" part of that, which then has the very clear meaning of
(something similar to 1 hour of walking, something similar to fighting, something similar to casting spells)
Actually, I would say the “similar” part pushes the natural interpretation towards A, as I would not say that 1 hour of walking is at all similar to (6 seconds of) fighting or (6 seconds of) casting spells. Rather, the similar suggests that all items in the list - walking, fighting, and casting spells - are considered similar, in that they are all adventuring activities, a period of 1 hour of which would interrupt a long rest.
Regarding Crawford, I alluded to appeals to authority as an option earlier. It's a reasonable basis for choosing an interpretation. Worse in my view than working from principles or preferences for how it plays. You've also at times mentioned "natural language" - I was thinking about that and I wondered if that might come down to something like where people are respectively from. What one group find natural, another often does not. I personally find the B reading more natural, but that is probably because I equate natural to some extent with the simplest reading, and B is structurally simpler with no peculiar system features.
Considering my entire point is “I’m pretty sure it is intentional that fighting alone cannot plausibly interrupt a long rest,” I think authority is the strongest point in support of my argument. We can bicker all day about which interpretation we prefer, but I don’t see much point to that. You run the game how you like, I’ll run it how I like. But I would say that Crawford’s tweet is pretty irrefutable proof in favor of my argument that it is intended by the developers that combat alone be implausible to interrupt a long rest with.