Nice theory, but not true.
I've saw the video and it looks like his head was decapitated by the missiles.
Look here
If you pause at 1:40 you can CLEARLY see that he did not duck his head back in to the lava.
Also there is the fact that Capcom themselves confirmed his death
http://www.gamegrep.com/news/23195-capcom_wesker_wont_be_in_future_resident_evil_games/
I liked Wesker just as much as the next guy, he was my favorite Resident Evil character next to Leon, but face it dude, Wesker is gone......but you always have Alex Wesker to look forward to. Hopefully he will be just as badass as Albert was.