Can someone please simplify the following 2 lines of code.

but still my first question: is about ending the if condition line with a ;

The if condition does not end with a semicolon. It is the statement to be executed when the test returns true that ends with a semicolon.

and starting with the {do something code} as if the if condition is still open.

I believe that this is the section of code that you are looking at

     do {
        inString[inCount] = Serial.read();       // get it
        if (inString [inCount] == INTERMINATOR) break;{
          (++inCount < INLENGTH);
          inString[inCount] = 0;  // null terminate the string
        }
      }
      while (Serial.available());             // wait for input

Here it is reformatted with added comments

do
{
  inString[inCount] = Serial.read();       // get it
  if (inString [inCount] == INTERMINATOR)
  {
    break;
  }
  {   //not needed
    (++inCount < INLENGTH);  //brackets not needed
    inString[inCount] = 0;  // null terminate the string
  }   //not needed
}
while (Serial.available());             // wait for input

And what about

(++inCount<inLength);

is it equivalent to?
if (inCount<inLength) inCount++;

No it is not equivalent, but the original code does not look it works anyway. It shows all the signs of having been revised without thought, hence the extra braces and brackets