I thought that you created an option to allow IPs to opt out of media viewer, but here you are saying that hovercards would be the only reader preference option. Am I wrong about media viewer?
Topic on Talk:Beta Features/Hovercards/preferences
No, you are not wrong. The difference is, media viewer can be re-enabled from commons image detail page. we don't have a common place to enable hovercard's and hence we came up with these options.