You don't need a TopMost window to cover the taskbar. Creating a borderless maximized window is enough:
public MainWindow() {
InitializeComponent();
this.WindowState = System.Windows.WindowState.Maximized;
this.ResizeMode = System.Windows.ResizeMode.NoResize;
}
You still need a way for the user to activate a window of another application. It isn't clear what you had in mind, but Alt+Tab works.