I like the idea of having commands for closing a working file or closing all working files and I can look into adding those for the next update.
As for navigation between working files: We have a very powerful tool for navigation called Navigate History. By default it is assigned to Ctrl + Tab and you can leave the Ctrl key pressed and click Tab to cycle through the list of files, similar how you can switch between windows on the OS.
Navigate history is not identical to what you are asking for because it also contains files that you opened that are not in working files. Can you give it a try and report back if it solves the navigation problem for you?
We feel that this is a more natural way of navigating, because you don’t have to worry about the order of working files. Instead you navigate across the list of most recently used files.
In our team we are so used to Navigate history that we cannot live without it anymore. Very often we would press Ctrl + Tab one or two times without even looking at the list that opens because we know that the file we want was either one or two history entries away.
Update
With the release of Visual Studio Code 0.5.0 there are now keyboard shortcuts to navigate back and forward in the list of working files. The shortcut is CMD + K + ↑ and CMD + K + ↓ (on Windows use the Ctrl key).