"suffocatingly narrow" or "narrow to the point of suffocating"?

I doubt whether there's any reliable objective measure to judge which locution is more common. Regardless, the safe thing to say here is that both sentences are acceptable.

The first may be preferable to some readers because of its greater clarity. The second sentence may be interpreted incorrectly if taken to mean: The confines are suffocating for (with the aim of establishing) the freedom of her spirit. Of course this isn't what's meant, but syntactically the gloss is available. Some quick readers may have to backtrack and reread the sentence in order to understand the idea correctly.

As for prepositions, we may use: to, for, or against.

My preferred rendering:

The confines of the system were suffocatingly narrow against the freedom of her spirit.