Quadrature Encoder:Interrupts: Reset Count
Is it possible to implement the reset of the main pulse count with a
  second interrupt as described?


Yes, and it's quite straightforward. Your index signal goes to pin 19,
which is interrupt 4 on the Mega, so you can simply



attachInterrupt(4, reset, RISING);


in setup(), where reset() is a function that resets the counter:

void reset()
{
    count = 0;
}



  I'm thinking there might be a problem here to implement this approach
  as the interrupts would have to be nested


In general you do not want to nest interrupts. If an interrupt is being
serviced when a second interrupt fires, the second one will just wait
for the first one to finish. This approach works fine as long as you do
not have interrupt handlers that take too long to execute.