#1 for summer, I think you will be fine with one cover, Top or Center removed, in all but the most extreeme condtions, such as steep hill climbing. I do think you should remove one. ( keep the bottom one on year-round... I got a rock chip down there that made my A/C freon leak out! )

#2 you can tell if you are "borderline" on too hot, but within limits, if you hear your electric radiator fans keep going after you turn the car off.

#3 there are two levels of over heat. Limited overheat, and severe. In limited overheat, your dash light will blink, and you may get a warning message, but the car will keep driving. In a severe overheat, I think the light is steady burning, and within seconds or minutes after that, the car will shut down at it won't let you drive at all ( untill it cools significantly ).

The fans spin all the time when the A/C is being used, but don't spin at all if the car is under about 100'C 212'F. They do have multiple speeds, and you may not hear them if they are on low.

How was your winter MPG with the covers?