hmm never mind. in case anyone else has this problem basically i tried a ton of things. but eventually i got it working by moving my Physx folder out of Program files. then i tried intalling the new Physx and it still gave me an error but the installation finished anyway. so then i copied the older Physx folders back in with the new one.
I ran into this issue too trying to play Nation Red, but I eventually got it solved by simply installing the old version of PhysX that created the .MSI file that the new PhysX install is attempting to find/remove. Once you do that, the install will now follow the same path but this time the .MSI file will actually exist and therefore your new PhysX installer will successfully remove any traces of the old versions, and complete its installation.
I know you said your way worked, but if anyone else is having this issue, my way for sure works too.