All that evidence you provided just shows that he has Las Plagas, which I am not arguing, he probably does. But none of it proves he doesn't have the Wesker virus which is part of the T-Virus family. There are more than enough clues to suggest that he has been infected with whatever variation of the T-virus Wesker has.
The report shows that he had his arm before this incident, also he has been with Wesker for some time, and he was reported dead at some point, very much like Wesker was. Plus visually that arm is nothing like the Plagas, it stands out during RE4 cause its so different.
This mutation:
http://youtube.com/watch?v=MmwjS6K9D2A
Looks like this one:
http://youtube.com/watch?v=32CiLGcZuNs (and I dont mean that its an arm, its how the monster mutates)
Or even this one:
http://youtube.com/watch?v=sVKamUj0fuQ
Basically choose any monster from past RE games and that arm mutation fits right in with all of them, it does not fit in with how Las Plagas operates.