Step 4 is incorrect. The 200 ohm resistor is shorted and the 12kohm resistor is in parallel with the 1Mohm one (assuming you're trying to calculate the thèvenin impedance across the 1Mohm terminals).
Finally, the 100Mohm resistor (I guess the op-amp input impedance) seems to be the load in your schematic. If so, you're calculating the equivalent impedance of your preceding circuit so that you can model it as a thèvenin voltage source in series with this impedance you're calculating. The 100Mohm will be the load.
Your result should be approx. a 12kohm thèvenin impedance.
The thèvenin equivalent voltage should be:
$$
V_{th} = V_{in}\frac{1M\Omega}{1M\Omega + 12k\Omega}
$$
Once you have voltage source and series impedance, you can then calculate what the voltage at the input of the op-amp will be. You'll see that it'll be your input voltage \$V_{in}\$ multiplied by a factor very close to 1.