How do I open an .edmx from Visual Studio 2015's Solution Explorer as XML and not in the WYSIWYG editor?

You cannot just double-click the filename. That gives the WYSIWYG tooling. Instead right-click on the .edmx in the Solution Explorer and pick "Open With..." which will give you some options. You can probably just pick "XML (Text) Editor" here to you get what you want. The option for "ADO.NET Entity Data Model Designer (Default)" in contrast gives the WYSIWYG (what you see is what you get) GUI tooling.