Blue, this may help you:
In my theme functions.php, I have added this action:
add_action('init', 'my_canvas_hotfix_remove_filters', 999);
The "hotfix" function is then:
function my_canvas_hotfix_remove_filters($query = NULL)
remove_filter('pre_get_posts', 'woo_exclude_categories_homepage', 10);
That allows the Posts 2 Posts admin panel to work. Whatever the woo_exclude_categories_homepage filter is, we don't use it and don't miss it.
With this in place, the front-end widget works *some* of the time, but we have not followed that up, as we do not use the front-end widget (we link posts with no public pages, and so just operate it through the APIs).
I have raised this as a ticket with WooThemes over a week ago, and not had any response yet. So I have no idea if this is something they see as their problem. What would be ideal is for WooThemes to talk to the developer of this plugin directly. One side, or both sides, is not playing fair with the WP query hooks, or perhaps this is a bug in core WP? Whatever it is, it is something deep inside the Canvas theme that I have not been able to dig out.