So I read a bit more on this topic, and thought I would post what I found in case anyone else asks this question. In the ANA member section under Resources / Patient Info booklets, there is a helpful booklet on improving balance. The following quote is found on page 16: "In some cases, persistent imbalance may be due to the poor or abnormal signal coming from residual vestibular nerve fibers on the AN side. This abnormal vestibular nerve signal may interfere with the brain’s interpretation of the signal coming from the other ear’s vestibular nerve." The solution to this problem, according to the booklet, is either to perform a labyrinthectomy, essentially removing the inner ear and taking out the entire vestibular nerve, or to try to treat it first using Gentamicin injections.
In an another article on vestibular nerve section, done to try to control balance issues for some conditions, the author (Timothy Haine, M.D.) states that "some of the fibers of the vestibular nerve run very close to the cochlear (hearing nerve)," and because of this they may not be completely cut out in the procedure. https://www.dizziness-and-balance.com/treatment/vn_section.html
If I'm reading these right, it seems that with the retrosig approach, there *could* be some residual vestibular nerve fibres left behind, and in the off chance they send out false signals, they *could* contribute to ongoing balance issues even as the good ear tries to take over. Not sure if it's worth worrying about, but it seems to fit with what the doctor I originally saw was telling me.