Qfyilyi

The sentence I'm trying to make is,

She would have done [this] without interceding circumstances;

or,

[this] is what she would have done if not for intervening circumstances.

The circumstances were forced on her, and outside her control.
What I'm using now ("If not for external/intervening circumstances") works, but the sentence feels clumsy. Is there a single word that work for this situation?

One possible thing to do is to mention what those circumstances are, so the reader knows exactly what the conditions are absent which she would have acted differently, then use the word "otherwise". So in the lead-up sentence you can do something like:

"She had been under intense pressure to keep quiet about the
matter."

then:

"Otherwise she would have raised her voice and let everyone know."

In other words, in keeping with your example:

Mention the "intervening circumstances"

then:

[this] is what she would have done "otherwise"

or

"otherwise" [this] is what she would have done.

"otherwise" can mean "had the circumstances been different", however the presence of such circumstances will need to be at least mentioned beforehand because "otherwise" obviously only makes sense with contrast to another statement or fact.

I can't afford it. Otherwise I'd buy it.

^^^^^^^^^^ Imagine omitting this first part.

If you're specifically looking for one word to mean either:

without interceding circumstances.

or

if not for intervening circumstances.

as opposed to my suggestion, I'm sorry I don't know any way. The only way I can think of is along the lines of:

There were intervening circumstances. Otherwise she would have
done [whatever].

Maybe there's a Latin term or something that can do this, but I really doubt it would be one word anyway.

The word complications can be so defined and seems to cover "intervening circumstances".

[this] is what she would have done if not for complications.

She would have done [this] without (had it not been for) complications.

https://www.thefreedictionary.com/Complications

Not an exact answer, but if the underlying issue is the "clumsy sounding" you might want to consider the use of "extenuating" circumstances instead of "interceding" or "intervening" since that's pretty much the exact idea you're trying to get across, is a common term, and sounds fine.

A situation or condition that provides an excuse for an action, as in Although Nancy missed three crucial rehearsals, there were extenuating circumstances, so she was not dismissed. This expression was originally legal terminology, denoting circumstances that partly excuse a crime and therefore call for less punishment or damages.

Other common usages would be:
"Due to extenuating circumstances, Nancy did [this]" meaning that Nancy did [this], but would have done something different if not for the circumstances.

You could also trying using the verb form (extenuate) but it's less common.

To extenuate is to make less of something or try to minimize its importance. The fact that you walked your little sister to school because she missed the bus might extenuate your teacher's response when you show up late.

See also "mitigating factors"

I don't know of a single word, some good idioms in this circumstance would be "all being well she would have.." or "if everything had gone smoothly she would have..."

I found this word that might work here -

perforce adverb

per·​force | pər-ˈfȯrs

1 : by force of circumstances or of necessity

2 obsolete : by physical coercion

Here are a few example sentences:

For a time Montrose retired, perforce, from public life.

The kings of England became perforce much more home-keeping sovereigns after 1204.

These images are perforce in black and white because there is no color at x-ray wavelengths.