1. Is there any advantage of having one and half years of experience? Or should I have more? Somebody told me that if anyone has more than two years of experience he'll get some additional scores.
That is not true. There are no such standard criteria. Moreover, if you apply now, you will be working for over two and a half years by the time you get an admit. These are the kinds of myths that you don't want to entertain. So, any work experience for any length of time will add to your strengths.
2. How difficult is the process from getting the GRE score to starting college? Is it possible for a person who's employed to handle the situations without help of any external agencies, like coaching centers?
YES! It most definitely is possible. However, you will need to approach the application process in a timely, professional manner and work out a detailed plan of action. Do be prepared to take a total of one or two weeks off over the course of a few months to take care of GRE preparation, the actual tests, obtaining reference letters, transcripts, etc. Moreover, there are no high quality coaching centers and training programs that can actually do this for you in a professional way.
3. If I am working, or about to start work before applying, what is an ideal time to take the TOEFL exam?
A GRE score is valid for five years and the TOEFL score for two years. I suppose most people take up GRE and TOEFL in the final year. If some of you are planning to work for more than a year, you should retake TOEFL.
It's better to take TOEFL as close as possible to the admission year.