Feed all the vertices to Newton and let collision be handled. Or if simple convex such as Box, sphere, ellipse would suffice you can calculate sizes yourself and feed them into Newton.
Sjef van Leeuwen translated the Newton tutorial to Irrlicht.NET, download it here http://irrlicht.sourceforge.net ...
You can't create a window on the client this way, ASP.NET runs server side.
If you want to host it on the client, you will need to encapsulate it in a browser control. like ActiveX and ship the dependend DLL's the the client's browser.
You are initializing the device on a different thread than the render loop. There are issues with that, OPENGL will then show a black screen. Try DirectX8 or 9 and it should work, or choose to run the control + irrlicht on the same thread.