So... I've completed the game and I have to admit that I was hoping that some point the orchards would fruit again, because I really like the bananas and mangoes! Is there anything that can cause this or does only happen once every blue moon with random event?

Explain a bit. What do you mean by "fruit again"....as in "restock the fruit" or "let you have more trees"?

All of the trees "restock" their fruit, it just takes a while. As far as I know, there is no way to get more trees in the orchard, or to "delete" the existing ones in order to get more.

Once the trees are harvested completely of fruit, how long does it take them to fruit again?

Honestly, I have no idea, I've never timed it, but they do fruit again, just more slowly than the initial harvest. (look at the top of the tree (or the bottom in the case of the banana tree) and you should see a fruit there when it's restocking itself)

Does anyone know if the trees ever return to full fruit?

They would if you didnt harvest them. If you have it on fast gameplay they regenerate about 100 food each every 45mins-1hour or so, usually after rain.

Good tip for getting some extra food. Play on fast and when it starts to rain set it to slow. The rain takes longer to finish and mushrooms tend to appear more at this time. So you can get more mushrooms for an extended period of time

I had my villagers fishing but they would still occasionally mosey on back to the orchard and pick fruit off the poor trees. I did a few rain dances and changed ALL of my farmers to builders. So now, slowly but surely, the fruit is coming back!

Thank you!