From the "Virus Memo," written by Dr. Birkin concerning the Experimental Virus given to Wesker (as a fulfillment of Project W)
"...Even in death, as long as some portion of the host remains, the virus can reconstruct the rest, and bring it back to life. During this process, there is a brief period where the host appears, for all intents and purposes, dead...."
So, canonically, Capcom has left open a way to resurrect Wesker, who still carried the above virus at the time of his second death, as long as some portion of him got blown clear of the lava (which could presumably cremate an apparent "dead" body and prevent resurrection).
Odd, that