Where can I change Icons for my App in VS2012 for Windows Phone 8?
How can I open WMAppManifest.xml in Visual Editor? Is it availble in Express edition?

The visual designer is available in VS2012Express.
If "View Designer" isn't opening it with the designer, try right clicking on it, selecting "Open With" and then choose "Windows Phone Manifest Designer".
If that option doesn't work or is missing it would suggest a problem with the install. The first step in trying to address this would be to repair the install of the SDK.