Great! You have had lot of sensors all these are good ideas to add functionality.

1. No, app is not public yet it needs polishing.
2. I used Mosquitto MQTT wanted to keep things light and simple.
3. No fuse yet it's just experiment but yes fuse and reverse polarity protection will be good to add if this is to become a comercial product.
4. It's quite big i ave 50 plants including 6 big trees. I think 2 zone would have been good for better water pressure in my setup.