You didn’t mention the symptoms of what the valve was doing, but here are a few things to check.
The valve may be installed correctly but not holding temperature because of a crossover problem somewhere in the house. Does this house have a recirculating system? If it does, you need to make sure the return line enters the bottom of the water heater and that there is a check valve between the pump in the return line and the water heater. If you don’t have a check valve located here you can get crossover in your system. To check for this, isolate the recirc system out of the flow loop and see if the valve problem still exists. If the problem at the valve goes away, the problem is with the installation of the recirc system. Other causes may be crossover caused by other shower valves in the house or even a stuck solenoid in the clothes washer.
As for the screws, try Grohe p/n 02697, they are longer. Try calling Grohe and telling them that you roughed in the valve within tolerances but the screws are too short, and you would like the screws listed above.