Q4) I've seen there is Windows support via a DirectShow plugin. Does that plugin work with HTML5 Audio or is it like Ogg Vorbis where the plugin works in most things but not IE with HTML5 audio? Sorry, not a Windows guy.
Q1: I use foobar2000 to add cover art to everything. This is windows specific and probably not what you are looking for. Although foobar2000 does run pretty well in Wine.Q4: LAV Filters support Opus just fine: http://forum.doom9.org/showthread.php?t=156191 DirectShow is in no way related to web browsers. If you want IE11 support, you'd probably need to find a flash applet which decodes Opus.
HTML5 audio tag with Opus has been supported in Firefox for a while now and I believe that Chrome and Opera now has support by default (otherwise it's a switch that needs to be turned on).