I hope there's a way to do this. I have a folder tree on the left and two lister windows side-by-side. I often sort folders by descending date and want to look inside without using directory navigation. If the folder tree could be sorted by date this would be easy.
Another way that would work is if I selected the drive in the folder tree, sorted the center lister by date, and then click on a folder there and have the contents display in the right window.
For the first part I think you're out of luck as I don't know of any way to change the default sort by name for the folder tree.
However you can do the second part easily enough. With Opus set in single pane mode/with folder tree present, in the folder tree click on your desired primary folder, then in the file pane sort the first level subfolders as you want (this is what you said you can do already). Once sorted the way you want, then in the center pane double-click on the desired subfolder you want to open it, and then press the |-> icon in the upper right corner of the center pane, to open that same folder in a newly created right pane. (a dual pane lister). Make that right pane active, and put it in flat view mode to see everything in the subfolders of the folder selected in the center pane.
As well as the |-> icon John mentions, you can also configure actions like double middle-button click to do the same thing, or create a hotkey or toolbar button. Shout if you want to and need help setting it up.