For scaling, I think you'll have to use the Viewport Sprite. Applying a Viewport to a Control I believe just gives you exactly that size looking in. It's useful for split screen games. I could be mistaken, but I'm not familiar with any options for forcing full resolution to this method.
For a Viewport Sprite, you add your Viewport to a Node2D instead of a Control. Set it's Viewport/Rect size to your desired full resolution. Then set Render Target/Enabled to true.
Add all the usual world items as children to it.
Then add a Viewport Sprite somewhere in the tree outside of the Viewport node. Assign it's Viewport to the one you created.
From there you can scale and transform the Viewport Sprite as you like.