The whole clone thing is kind of easily dismissed, as Wesker's rapid change in personality was not due to RE5 alone, it started in UC with Wesker's latest voice actor, D.C. Douglas, before that he seemed to have the same personality throughout...
Let me just recap everything else, he was covered in Uroboros, not internally, but externally, his virus could have worn out at anypoint down there in the volcano, the two RPGs exploding right next to him might have triggered that, he was standing in lava, it wasn't burning his legs off, but obviously he was in pain. I am positive that if he were in the lava longer it would have taken its toll.
Let me just recap everything else, he was covered in Uroboros, not internally, but externally, his virus could have worn out at anypoint down there in the volcano, the two RPGs exploding right next to him might have triggered that, he was standing in lava, it wasn't burning his legs off, but obviously he was in pain. I am positive that if he were in the lava longer it would have taken its toll.