Double predicate, but not conjunction
I am editing for a writer that consistently uses a form that I'm not sure is correct or not. What is this style of sentence structure, is it good or bad, and how do I make it better?
1) "She felt her eyes creak open as consciousness seeped into her, producing a soft gasp from her tired lungs."
2) "She stumbled back in horror, shielding her crying child."
3) "She leaned in close, her face as kind as ever."
Etc.
A bit of nomenclature may be helpful here. In some grammars, a (clause) has a subject and a predicate, the latter containing the verb and adjuncts not modifying the components of the subject. Your examples contain one clause apiece, call it a main or independent clause if you wish:
(She felt her eyes creak open as consciousness seeped into her), producing a soft gasp from her tired lungs.
(She stumbled back in horror), shielding her crying child.
(She leaned in close), her face [being] as kind as ever.
Each part outside the parens contains a present participle, but no subject. And none of them directly modifies either the subject or predicate of its associated main clause, but is associated with the entire thing. For instance, in the first example, producing a soft gasp partly describes the woman in question like a noun modifier, partly describes when the woman's eyes opened like a temporal adverbial modifier, and partly tells how she came into consciousness like an adverbial modifier of manner. In fact, the participial construct stands independent of the grammar of the main clause. Thus it is called an absolute construction, or a nominate absolute, in recognition of its association with the subject of the clause.
Applying the same analysis to the other examples, shielding and the elided being, is left as an exercise for the interested reader.
Notice that you can make the modifications explicit by rewording the sentences with dependent clauses:
1a. The woman who produced a soft gasp felt her eyes creak open. (subject is modified)
1b. She felt her eyes creak open as she produced a soft gasp. (verb is modified)
Or you can replace the absolute with a conjoined independent clause:
1c. She felt her eyes creak open, and she then produced a soft gasp.
But all these the options are grammatical, making the judgments of good, bad, and better a matter of style.
The first two are examples of non-finite clauses functioning as supplementary adjuncts. They don't fit into any particular semantic category; instead they provide useful but non-essential information about the situation, in particular the subject they relate to (in this case "she"). They are not tightly integrated into the structure of the clause (as modifiers would be), but are loosely attached. Notice how they are separated from the rest of the clause by a comma; in speech they would be marked off by a slight pause.
Your third example is slightly different in that it is a verbless clause, but it’s still a supplementary adjunct with "her face" as subject.
This wikipedia article calls those kinds of sentence structures complex sentences:
A complex sentence consists of one independent clause and at least one dependent clause. - wikipedia
Richard Nordquist produced an article about adverbial clauses, saying:
The name "adverbial" suggests that adverbial clauses modify verbs; but they modify whole clauses ... . Their other key property is that they are adjuncts, since they are typically optional constituents in sentences. They are traditionally classified according to their meaning, for example adverbial clauses of reason, time, concession, manner or condition ... .
According to these, your first example is an adverbial clauses of manner - it describes how "consciousness seeped into her". I'm a little less certain of the second, but it seems to be the same, describing an aspect how she "stumbled back".
I'm even less certain of the third. It may be a adjectival clause that provides information on her intentions when she "leaned in close".
In any case, all three sentences are fine grammatically and work well in the context of telling a story. Any modifications would depend on the effect you're trying to achieve.