Power-fail safe configuration

My RPi HMI device must be more like embedded than the OS-driven system. The close use case is kiosk (a totally new English word for me, had to check a dictionary!)

What does it mean and what have I to do with it?

RS-485 circuitry and software control

My RPi-as-HMI communicates with the MODBUS Slave upon the RS-485 line. It's a well-known fact that there is a strong necessity to use isolated solutions in the industry, medical devices and for a noisy environment (Intersil note on the issue is just an example). So I have to follow the trend.
I want to note the hardware and software sides of the Raspberry-RS-485 topic.
Below I note, step-by-step, my research. Most of it happened to be in vain. The result is simple. But I want to keep all branches, just for myself.

Hardware 

Here is an extended illustration of the RPi-as-HMI structure:

Raspberry Pi as MODBUS HMI panel (RPi-as-HMI)

I need the HMI (an operator panel) for a device communicating as a MODBUS Slave. I used the panels of SH-300 class and I want to create a prototype for the new generation, based on the touchscreen monitors.
Basically, the system consists of 2 parts, placed at the distance from hand-reach to 100-200 meters:

I'm going to use Raspberry Pi Zero, but I want to be able to use the RPi 3 with the HDMI display the mouse-keyboard at the development stage. 
The preliminary experiments show a good chance of success. Now I'm in the state of developing the software for Raspi and adapting the firmware of the Slave. And, since my "tries and exceptions" have consumed a lot of time and efforts, I want to note my steps for the future use.