Hi Shah007,
1) My friend's had some family issues and so there was a possibility that she would need to go out of the country. So she contacted the local Congressman to expedite the rescheduling process since she did not get the interview letter even 2 weeks after the Infopass appointment.
2) Calling the 1-800 number does not help. The interview is scheduled by the local USCIS District Office. So if you wish to reschedule, just follow the instructions on the interview letter. It asks you to return the Interview letter along with a written explanation why you cannot attend the interview and requesting them to reschedule. In fact I called the 1-800 number just to confirm after I had sent the letter and they told me that they cannot help. They just asked me to wait for the Interview letter with the rescheduled date.
3) If you can change the dates of your trip, there are no issues at all. In my case, I was in no rush and was ready to wait for a month or two for the interview date to be rescheduled. Luckily, by the time I got back from my trip, the rescheduled interview letter was waiting for me