As far as the identity of the people who founded the IVC is concerned, there is still nothing to connect Tamils/ Dravidians to the IVC, and even less than nothing, if that is possible, to connect the Japanese to the IVC.
However, regarding the possibility of a connection between Japanese and Tamil, it needs to be explored gingerly by linguists, and it should be linguists with an open mind. The biggest thing favouring this unexpected link is that the Japanese-Altaic link has not been established yet, it is a speculation. HIM Akihito told the Hungarians that a connection between Japanese and Magyar cannot be ruled out. As you know, ultimately the Finno-Ugric languages, of which Magyar is one half, has an Altaic descent (until a few generations ago, Finnish speakers and Magyar speakers could understand each other, they no longer can, and it is uncannily like the ongoing separation between Tamil and Malayalam).
My suggestion: leave the IVC ethnicity alone. Concentrate on the possible language link.
Thank you.