I edited the extension.vsixmanifest file in the RCPackage Extension to change v14 to v15 and got the installer to install the extension just fine, it did however pop up a warning that it might make VS2017 unstable. Is that just generic or is there some reason I should not be used that extension in VS2017?
Yes, we could not just change the manifest to add support for VS2017 because Microsoft warns against that (and also warns the user, as you can see). So, we rewrote the extension to use version 3 of the manifest, which requires also all items to be changed.
Start a new project with the vs2017 RhinoCommon or GH template, then change the references to Rhino 5 RhinoCommon.dll. Alternatively, you can use the RhinoCommon Nuget Package for Rhino 5.
Rhino 6 RhinoCommon template also references RhinoUI.dll and Eto.dll. You’ll need to remove those.