I think you have a colorable case for doing the retreat on a B1/B2 visa, but given that you have been denied the visa for this very trip I suggest consulting an immigration lawyer before applying again, so that you can get a better impression of what the problem is (based upon the full facts) and discuss possible paths to approval or other options.
It is also possible that the concern was that you lack sufficient ties to India for the immigration officer to be reasonably comfortable that you will return home at the end of your authorized stay.

