* Is it possible to mask or auto-hide the tree on the left? I tried to drag the divider but to no avail. Is it skin dependant?
Auto-hide? Not as far as I know. But like Jim said, you can collapse it all the way to the left by clicking the little triangle that's just to the left of the divider bar. Note that these pairs of triangles are in most dividers in MC, and can be used to fully hide or fully expand those divided areas. Very convenient actually.
Panel 3a (top): tracklisting of the selected cover thumbnail (so I can choose which tracks I want to enqueue to my current playlist)
I know why you're trying to do this and I've thought of it myself before. It's not supported as far as I know. But, you can toggle your list style with hot keys. For instance <control><shift>U will show you the details view style. <control><shift>P will show you the album thumbnails view style. So you can toggle back and forth when you want to see details, or just album thumbnails.
Panel 3b (bottom): current playlist
Now this you can do! Use Split Views. Split Views allow you to split the main screen (middle) into several horizontal or vertical slices. You can control the size of each one, and what is displayed in it. Play around with it some: View > Split View
Brian.