In one sitting, all at once (AmE usage)

Question

Will it be okay to use "all at once" instead of "in one sitting"? I mean in AmE what would sound more natural? Do they have a slight difference in their meaning?

1. I read the book in one sitting/all at once..

2. I watched an entire season in one sitting/all at once. (I know that "binge watching" is better, but I wanted to know about "all at once")

3. I ate the whole cake in one sitting/all at once.

4. I drank the whole bottle of water in one sitting/all at once.

5. I typed the whole script out in one sitting/all at once

Answer 1

The two phrases have similar meanings but are not synonymous.

In one sitting - During one uninterrupted period of time. Source: The Free Dictionary (Idioms)

All at once (in the literal sense) - Simultaneously (Happening, existing, or done at the same time.) Source: The Free Dictionary (Idioms)

"All at once" can also be used figuratively to mean suddenly (same source)

In my opinion (although I'm BrE, hence my hesitation to answer), non of the phrases you have provided sound natural with "all at once". However, there is a case that can be made for them using the phrase in a figurative sense.

I.e. You didn't literally drink the bottle of water in one go, but you drank it so quickly, that the situation was afterwards described as such.

Here are some suitable sentences which could use all at once in a literal manner:

We can't all get up at once. - Multiple people

The memories came flooding back to me all at once. - Multiple memories

Answer 2

I think "all at once" may not be suitable for many of these sentences. Macmillan dictionary describes it as "very suddenly".

Following userr268429's comment, I stand corrected, "in one sitting" is idiomatic for doing things without stopping.

Another adequate phrase for these examples is at one fell swoop, which was likely coined by Shakespeare.