Unfortunately, there's currently no workaround for users who are uploaded by AD. Webex will only use their AD name. Webex will allow the name change as you've found, but won't save/show the name change since it syncs with AD. If the company AD requirements require the user's legal name, maybe a feature request would be able to override the AD name in the user's profile.
https://ciscocollabcustomer.ideas.aha.io/