Where I believe we landed last time we chewed over these lines is that the words in the book are literally ambiguous: they really can be taken either way. Many people agreed that 600 rounds of combat can't be what is intended, notwithstanding JC's tweet.
What do you mean by “600 rounds of combat can’t be what’s intended”? Surely you aren’t suggesting that the intent is for 600 rounds of combat to
not break a rest?
With that in mind, I feel the most plausible approaches are either
- You say that it is a collection of activities together that break a rest, which is the approach you have outlined
Any combination of the activities listed, which together add up to one hour of activity, breaks the rest. That would include 600 rounds of combat, if such a thing actually happened. A much smaller amount of combat, combined with other activities, is of course the more likely scenario to occur in actual play, but the fact that the rule happens to also cover a situation that is not likely to come up in actual play is not evidence against that interpretation.
2. You say that any amount of combat, any spellcasting, an hour of walking, breaks a rest
I find this reading - although technically grammatically plausible - to be extremely unintuitive, and therefore doubt it is the intended interpretation. Jeremy Crawford’s tweet seems to confirm it was indeed not the intent of the writers.
1. ends up requiring additional words because we have to rule out something that remains included under it, which is 600 rounds of combat.
Why do we have to rule that option out? I think that (in the unlikely event that it were to actually happen) 600 rounds of combat absolutely should break a long rest.
In fact, 600 rounds of combat breaks a long rest under either interpretation, so I don’t see why it being covered by 1 is relevant. Both interpretations have that fact in common.
Or if we decide that it must be always a mix of activities, then we are also ruling out exactly 1 hour of walking, which can't really be what we want.
Why would we decide that? Is exactly 1 hour of walking “at least one hour of walking, combat, spellcasting, or other strenuous activity”? Yes, it is, therefore it breaks a long rest.
Thus, from the words written in the book, I believe there are better grounds to go with 2. than 1. If a DM prefers to put an hour of fighting on the same footing as an hour of walking - which amounts to saying that they prefer rests in their game to be very hard to interrupt - then they have JC's tweet to back them up.
What on earth do you mean “put an hour of fighting on the same footing as an hour of walking”? Just because both scenarios are covered by the same rule doesn’t mean they are equal in any way.