I have 3 different versions of SW installed in parallel. After I "installed" the API SDK for one, it would not let me install it for the other versions. This started me digging around to understand what the SDK really consisted of.
It seems that in reality, the *.dll's that are called when using the API are already installed with SW. In addition, you do not actually have to install the API SDK. You can just use 7 Zip to dig the Add In templates out of the archive folders, or write your own from scratch. You do not actually "need" the API SDK at all. Also, if you look at the Visual Studio Templates (for example if you use 7 Zip to look into swcsharpaddin.zip), you will see that the templates are exactly the same, and have not changed since 2010. For instance, the C# AddIn Template from SW 2011 is the exact same as 2018.
So it seems that all the SDK really is, is a bunch of source code for example programs in C#, VB, and C++ .NET.
Brant, you are right, when you install API SDK the following happens
C#, C++, VB.NET templates get copied to the project locations
SWWizard dll which is used in templates is added to GAC
The SDK is not required to use API and write add-ins and stand-alones. You can simply get interops (for managed code) or tlbs (for unmanaged) from installation folder and develop applications for SOLIDWORKS.