your code doesn't compile... and there are too many things wrong to comment exhaustively until you get that right.
but a few pointers:
Serial.readBytes() returns the number of characters placed in the buffer. A 0 means no valid data was found.
so as long as you enter one byte, you will get the same result here:
atof(incomingByte);
it seems like you are expecting more, trying get nine bytes...
Serial.readBytes(buffer, 9);