Hello again,
To answer your two questions more directly...
1.
You should see 1.033v on the output (1.035 is close enough) because for DC the circuit functions as a subtractor where it subtracts part of the two applied voltages vctl and vset. There are two gains to consider, the gain for vset and the gain for vctl. When vset is 0v that puts one half of the 1.5v battery voltage at the non inverting terminal. Since the gain for the non inverting terminal is R1/R2+1, the gain is 2, so with vset equal to 0v that puts 0.75 volts at the non inverting terminal and with a gain of 2 that puts 1.5v at the output for that input only. But then the input at vctl is 0.467v and the gain for the inverting part is only 1, so that puts -0.467 at the output for the inverting part only. Adding the non inverting part and the inverting part gives us 1.5-0.467 which comes out to exactly 1.033 volts. With a little input offset it will be slightly different.
So in finding the gain for the two inputs first calculate the voltage at the non inverting terminal, then multiply that by 2 and that is Vout1. Then find the output due to the vctl input, and that will be -1 times whatever vctl is, and call that Vout2. Then add Vout1 and Vout2 and that is the actual output.
A differential amplifier does NOT make the output equal to either supply rail unless one or both inputs are over driven. You might be thinking of a comparator circuit, which does exactly what you thought this does.
2.
The text, "Gain=100dB" means that the internal gain of the op amp is 100dB. That's the open loop again of the op amp.