No, it is necessary because if you remove “period you’re left with the sentence “ If the rest is interrupted by strenuous activity—at least 1 hour of walking, fighting, casting spells, or similar adventuring activity—the characters must begin the rest again to gain any benefit from it.” Which would actually make B the more natural interpretation because the aside would seem to be defining “strenuous activity” rather than “a period of strenuous activity.”
That seems a rather round-about approach. Just replace "a period" with "at least 1 hour".
You have not demonstrated how that’s a problem. I don’t think it is.
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!?
But B has the problem of producing the absurd conditions of any amount of “adventuring activity” interrupting the rest
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)
if A was the intended meaning, how would one word that with similar clarity, without making the sentence unwieldy
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."
It seems to me that if you consider the existence of a case where 600 rounds of combat can break a short rest a “sacrifice,” it is you who has a “preexisting preference for play” that you are trying to preserve with your tortured interpretation of the sentence.
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?
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.