Displaying a widget in sidebar only when no other widgets rendered?

I not 100% clear what your question is so hopefully this can point you in the right direction for conditional widgets. I will give you 3 options.

1. is_active_widget checks if widgets are active based in the widget ID.

• http://codex.wordpress.org/Function_Reference/is_active_widget

2. Using is_active_sidebar and creating more sidebars usually gives you better control over widget output.

• http://codex.wordpress.org/Function_Reference/register_sidebar
• http://codex.wordpress.org/Function_Reference/is_active_sidebar

3. Use a plugin, there are several plugins that have post or conditional logic built into them.

• http://wordpress.org/extend/plugins/display-widgets/
• http://wordpress.org/extend/plugins/widget-logic/
• http://wordpress.org/extend/plugins/dynamic-widgets/

4. Try to use the widget_display_callback filter for example:

add_filter('widget_display_callback','your_custom_function',10,3);