I don't see why not, as .NET 4.5 isn't an upgrade to new CLR, but just to be clear we didn't yet decided anything.
1) Any plans for moving to 4.5 .NET framework for major 5.0 release ?
I think that Denis explained/disclosed something about this in one of your posts, we won't shutdown WebForms project, we also can't disclose anything about MVC product as explained here
2) What's after 5.0 ? What is the future of monox after 5.0 ? more releases or other MVC version based project will be given importance more as compared to monox in releases ?
We can't disclose that yet, it too early as everything is planing stage.
3) When you will start the development work for MVC version based project ? Any approx idea for release ? like in mid of 2013 ? or end of 2013 or beginning of 2014 ? Would appreciate if we get an approx idea...