Debasis,
Let me get an extract from the site that you gave:
"How do I know if I need an ITIN?
If you must file a U.S. tax return or you are listed on a tax return as a spouse or a dependent and you do not have, and cannot obtain, a valid SSN you must apply for an ITIN. "
1. So ITIN can be requested for any one meeting the above criteria.
2. As far as getting someone's signature - well I believe that it is possible to get a signed form from India - postal service does exist. (if one applies brain, it is possible to think beyond forgery ... no offence intended).
3. Please excuse my ignorance, but isn't even if one wants to file as married filing seperately, ITIN/SSN required?
If you want to learn further ... plz do talk to a CPA or call IRS. What I am advising is what others have successfully and legitimately (per IRS) done.