BOAC:
The area along their route has radar coverage.
They went off frequency to obtain a runway conditions report. So, if or when the ARTCC's EMASW went off ATC couldn't contact them in the brief time before impact.
The remainder of your questions will have to await release of the radar, ATC communications and (hopefully) the CVR data.
Last edited by aterpster; 15th Mar 2013 at 13:30.