I had bell's palsy nearly 10 years ago with approx. 85% strength regained... is it still possible to work on the muscles to get a symmetrical smile?

Yes: Many patients with bell's palsy regain most if not all their function. 70% of patients recover completely within 6 months without treatment. Injection of botulinum toxin a, surgical reconstruction, acupuncture (big trial in korea started to see if works) may help. Patients may be referred to physiotherapists as well, although there is no evidence favoring one intervention over another yet.

Perhaps: There are a number of ways to balance the facial movements after a bell's palsy. The most common is to have some Botox injections to even out the facial movements. Eyelid and facial surgery sometimes is helpful as well. You should see a qualified facial plastic surgeon for advice.