Exactly what is "low heat" for making cheese sauce?
Cheese have enzymes. Enzymes break quickly and easily at 150F/65 C. 50 Celsius for a short period of time should suffice to melt you cheese without breaking the protein structures.

I have committed the same mistake many times. Actually the fist day my in laws where over i tried to impress them with homemade pasta and cheese sauce. My Cheese sauce clumped.   My back up plan is to add milk tomato sauce put it on the blender until it liquifies.