Is there a way to expand the Map List Widget when the application opens?
You have to add "Expand" to require and function, then create an element/widget object in your script. Then you have to create an expand object that calls the element/object you want to expand. Here's an example from a JS API script I wrote for a webapp:
Here's an example from ESRI
I am working directly in AGOL. If I understand correctly this feature requires the developers edition. This feature is available in WAB.