ESC signal standard and PWM frequency
You made a few errors in your code:

ESC.attach(9,100,2000); should be ESC.attach(9,1000,2000);, so, 1000 instead of 100.

and

potValue = map(potValue, 0, 1023, 1000, 2000); should be potValue = map(potValue, 0, 1023, 0, 180);
because the next line, the command  writeMicroseconds(angle) only accepts a value to write to the servo, from 0 to 180

Have a look how servo control works:




  2b. What if we send 1.5/2 ms for every 10ms signal to ESC? Does that represent 50%, or some value between 25% to 50%? Does it represent 100% operation?


You should send a pulse each 20 ms, because that is how (this) servo signal/BLDC motor control signal is defined. You cannot change that to 10 ms.
For your convenience: 

  0 degrees corresponds a pulse witdh of 1.00 ms corresponds to   0% of max RPM*
 45 degrees corresponds a pulse witdh of 1.25 ms corresponds to  25% of max RPM
 90 degrees corresponds a pulse witdh of 1.50 ms corresponds to  50% of max RPM
135 degrees corresponds a pulse witdh of 1.75 ms corresponds to  75% of max RPM
180 degrees corresponds a pulse witdh of 2.00 ms corresponds to 100% of max RPM

*or 1% (I'm not sure how it is implemented)



  2a. Can we send 5V constant signal to ESC?


This pulse width should be between 1 ms and 2 ms. A pulsed signal is not a constant signal, so, no, you cannot send a 5V constant signal.
That's also the reason the min and max values of ESC.attach(9,1000,2000); should be 1000 resp. 2000.