Nokia Xpress is available for Lumia[1]. Just like Apple doesn't allow alternate browser engines on iOS, but it allows Opera Mini[2] because Opera Mini isn't a full web browser, it's a client for Opera's transcoding service. Nokia Xpress isn't a full web browser, it doesn't execute JavaScript, it is a client to Nokia's transcoding service.

[1] http://betalabs.nokia.com/trials/nokia-xpress-internet

[2] https://itunes.apple.com/us/app/opera-mini-web-browser/id363...

Yes, they allow alternative browsers. I've seen web browsers on the marketplace (SurfCube, for example) completely functional. What I'm not so sure is if they allow browsers using other rendering engines than Trident (i.e., they don't use the native WebBrowser control).