I've never used opera-mini but if it's a server-side render then push over to the phone, there is no reason they couldn't port that to WP.. need to put pressure on opera for that, not MS (though with enough demand, MS could ask opera themselves).
like iOS, WP does not let you use any browser other than their own engine. Even chrome for iOS is safari with a skin.. Chrome could very well (and opera, and firefox, and whoever else) come to WP and just wrap their APIs and features to the trident rendering engine. Just need to get the companies to do the work which they don't want to do for ~4% US market share. Now if WP8.1 and Cortana can bump us up to like 7%... then we might see a lot more.. but WP is starting to be more of a common name for people. They might not know all about WP but they know it is a thing which is a start, albeit slower than i'd like.