Since these are all hypotheses, and no one has cracked the script yet for good, (at least where in all are in consent), I would suggest that these are looked from the Proto-Dravidian hypothesis as well. See the following links: http://www.hindu.com/mag/2007/02/04/stories/2007020400260500... The above is Iravadham Mahadevan's view.

The following two are Asko Parpola's contributions:

http://www.harappa.com/script/parpola0.html http://www.amazon.com/Deciphering-Indus-Script-Asko-Parpola/...