It should work, but it can be more complicated if something has taken over one of the file types via the Open With system in Windows, as it overrides the type.
In some cases you might need to edit the registry directly to change those icons, if you want it done at the filetype level (i.e. so it affects everything in Windows, not just Opus).
Opus has that as well. I think it's on by default. More detail about what you mean might help understand exactly what you want. But please make a separate thread for it (Ask one question per thread).
In Opus terminology, the "lister" is the whole window. The main panel within it that shows the current directory is the "file display", where there can be one or two of them side-by-side within a single lister/window.
If I understand correctly, you have a dual-file-display window which is active, and you want to change which file display has focus (and active/source states) within it, by moving the mouse wheel over it?
That should be easy to add as an option. We'll put that on our list. Thanks for the suggestion! (Edit: Added in 13.10.2.)