Hi,
If I download the newer Wifi lib into my Libraries folder in my sketchbook, will it override the default Wifi lib in the base Arduino installation?
A quick test appears to show that the base library takes precedence. You could add a line like this into the newer one and see if the error appears:
#error newer library
The IDE's way of using libraries seems a bit mysterious but you are not obliged to stick to the given names. If you name the new WiFiNew and call for that in your sketch, that is what you use, and you get to retain the old library as well.