are the phrases that begin with "unable" in these sentences considered something like participle phrases to add extra information to the subject of the main clause?
She fluffed the pillow and changed position, unable to sleep.
We both looked at her intently, unable to respond.
She fluffed the pillow and changed position, [unable to sleep].
We both looked at her intently, [unable to respond].
No: "unable" is an adjective functioning as head of the adjective phrases "unable to sleep" and "unable to respond". Within the adjective phrases, the infinitival clauses "to sleep" and "to respond" are complements of the adjective "unable".
Both adjective phrases are best analysed as predicative adjuncts: predicative because they relate to predicands, i.e. "she" and "we", and adjuncts because they are supplements in clause structure.
Note that infinitives are not participles, so they cannot head participle phrases (or clauses).
