Sentence in question is:
The stool wobbled under her when he took her hands, the one leg shorter than the others that his father hadn't mended in fifteen years gone past.
Test the sentence by removing the adverbial phrase:
The stool wobbled under her
Let's take out the prepositional phrase too.
The stool wobbled
No, that still looks right. Am I misremembering my grammar that badly?
...I think I need a new career...