You already can create a batch file that opens the first or second file parameter in notepad++. Add this as compare tool in SmartGit.

What specific topics should be covered?

Yes, you are right. The Working Tree window only can show files of the Working Tree.

Until implemented in the Log window, you may use the Working Tree window which supports showing the files non-recursively.

Double-click in the Files view also invokes Show Changes. Because the File compare tools only support files, it needs to be triggered on the file level, not on the commit level.

Is the current bold text not easy to spot?

In SmartGit you don't need to drag/drop to see a diff. Just select the 2 commits you want to diff. Then open the external file compare from the changed files in the Files view.

You can already right-click a repository and open a Git shell (or Terminal depending on the platform) where you can invoke the Git commands.

In the next 20.1 build "Nautilus" will be replaced with "Files (Nautilus)".