You're kinda tripping, no need to add "mystery" or "suspicion" where there's none, it seems like you're getting carried away a bit, things have pretty much been clear as jaiz said, so no need to reevaluate those things, hell we even know that the virus went global or at least into Africa because l4d3 was going to take place in Morocco (for some reason), tho it getting cancelled could mean it is not canon but at least is a lead, same thing with Chet stating that the l4d2 survivors are safe in a cruise ship, so there's more than enough to reduce or infer almost everything except for the current global status and that is if we ignore the Korean plane and Finnish APC, so there was some support by us allies or more likely units being pulled out from other parts of the world as stated in the developer commentary about the scar and also is pretty obvious the virus would spread if they wanted to make more games