There should be no problem with UE4 not being installed on the C: drive. On my computer, I have UE4 on the D: drive with Visual Studio installed on the C: drive and haven’t run into any problems with that setup. There also should not be any problems caused by having both UE4 and Unity 5 installed simultaneously.
Unfortunately we are still somewhat stumped by the original issue. We have not been able to determine what path Visual Studio is looking for here, or why it is unable to find it. Just on a hunch, when you reinstalled Visual Studio, did you make sure to install the C++ files? I believe VS 2015 does not include these by default.