I'm experimenting with your Posts 2 Posts code in two ways: one via your P2P-Core code in a plugin and the other in a separate instance of WordPress using your plugin proper. I've found that when using your P2P-Core, that 1. the p2p_init hook doesn't exist and 2. In order to have a query work, you have to explicitly set connected direction in the query. Otherwise, the query returns 0 results, even when there should be.
Conversely, for some reason if I explicitly set the connected_direction to any in a query done while using the actual plugin, I get zero results as well.