Windows SDK 8: Confusion Regarding Install Location, folder nomenclature and WindowsSdkDir_old env variable

Question

While working with Visual Studio 2012 build system, unlike previous SDK Versions, I realized, the Windows SDK 8 gets installed into two separate locations

C:\Program Files (x86)\Windows Kits\8.0\ - Everything sans .NET 4.5 

as mentioned in

HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Microsoft SDKs\Windows\v8.0\InstallationFolder

and

C:\Program Files (x86)\Microsoft SDKs\Windows\v8.0A\

as mentioned in

HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Microsoft SDKs\Windows\v8.0A\InstallationFolder

It seems Microsoft have segregated the Core SDK from .NET Framework

I also tried to correlate with how

 *    %VS_HOME%\VC\bin\vcvars32.bat
 *    %VS_HOME%\VC\bin\x86_amd64\vcvarsx86_amd64.bat
 *    %VS_HOME%\VC\bin\amd64\vcvarsamd64.bat

is working now, and it seems, it refers the .NET 4.5 Framework Location with WindowsSdkDir_old

Other interesting things I realized were

1. %WindowsSdkDir%\Include has three hierarchy: Shared, um and WinRT
2. Windows SDK got per-installed with VS 2010 but with VS 2012 I had to install it separately.

SO my question is

1. Is there any specific reason to separate .NET 4.5 from the core SDK?
2. What is the reason to refer the .NET 4.5 Install Location as WindowsSdkDir_old
3. Why Windows SDK 8.0A is not part of VS 2012 Install Package?
4. Can I continue to use Windows SDK 7.1A with VS 2012?
5. What is the significance of the include path %WindowsSdkDir%\Include[Shared| um | WInRT] ?

Answer 1

In order...

1. .NET is built by a different internal organization (Developer Division); the SDK is built by Windows.
2. Don't know
3. Visual studio never installed any Windows SDK. There are gigabytes of samples & tools in the SDK that aren't part of Visual Studio.
4. Yes, previous builds of the Windows SDK will work fine with VS 2012.
5. Not sure about um, but WinRT files are specific to the Windows Runtime, and can't be (easily) consumed by desktop applications. It's best to keep them separate.