Bees or even gnats can pollinate brugmansia, the one you cut off will not continue to grow. They do not open up by themselves but they will eventually drop of the stem if left to themselves. The pods mature fairly quickly in Florida compared to the northern states, I pick mine when the pod starts to get soft and the stem holding it turns brown, it seems that mine ripen in 2 - 3 months. When they are ripe open them up and let them dry out on a paper plate before storing them. The sooner you plant them the faster they will germinate. Hope this helps.
Connie from Indiantown, Florida