This is not a full answer to your question -- but if you remove Bows (and arrows) from your Mage's inventory, and supply them with Staffs -- they will have no option but to use Magic, seems to me.
I have had Followers who used both Bows and Staffs -- but currently Aranea (strengthened with Dragonscale armor) is my Follower and she never uses anything but Magic. She is so good with
Magic that she takes care of Forsworn Briarhearts all by herself.