Grammar: the difference between "be + about + Ving" and "be + about + to V"?

“The mother was about fainting” is more of a colloquialism, not the correct form for use in writing.

@augustlan OK, thank you.

Either “The mother was fainting” or “The mother was about to faint”.

The first thing that came to mind when I saw “be + about + Ving” was the colloqualism “I’m all about…”, with the meaning “I really like…”

For example, someone who really likes dancing might say “I’m all about dancing!”
This is often used with nouns as well: “I’m all about the movies!”
Just like if a book focuses on the subject of puppies, we say it’s about puppies.

I’m not sure if this is what you meant, but I’ve never seen about + verbing used in the way you cited in your question.

“about” can be confusing because it implies “subject matter.” “About to” has a different meaning, suggesting something is likely to happen.
The first usage:
The movie was about wartime Europe.
The mother was about keeping things tidy.
Thus it is confusing to say The mother was about fainting, and it is less confusing to say The mother was about to faint.
Alternatively we could rephrase it “The mother nearly fainted.”
It is also confusing in English however to say “The mother,” because the natural response is “what mother?” Usually we would indicate something more unique such as “my mother,” “Charlie’s mother,” or “The pregnant woman.”

If you say “about fainting” I am pretty sure most people would look at you funny, unless mother was deeply interested in fainting and made it her life’s work to understand it.

Thank you all. Makes sense.