Good morning, Balaji:
Most of your questions are those that only PMI can answer. With respect to your second question, I suspect there is a "grace period" with respect to an audit should it occur. That is, if you have all of your information in by the date, then your worksheet is selected for an audit, your credential will not expire during that time.
With respect to your first question (Possibility of an audit). The probability is not non-zero. I suspect that PMI has a metric for audit selection in accordance with their professional status. What that metric is will be a closely guarded secret within PMI's Professional Development section so people do not game the system. I do know from my other professional certification organization, that audits are randomly selected at about a 10% or so rate, with an exception for worksheets that appear to have irregularities associated with them.
With respect to your third question, (what will be expected). You will need receipts for everything. For example, if you traveled to a professional meeting you will need proof of travel (airline receipt, hotel receipt, meeting registration, etc.) or if you took a course, receipt that you paid for the course and/or a certificate of completion. For "free PDUs" it has been suggested that you have some kind of spreadsheet with the course title, time, and a short summary of what you got out if it.
I hope that helps. I might recommend that, in the future, that you don't wait until the last minute to get PDUs in. It's been my experience that something will always get in the way of completing the last few requirements!
Harry