@Skallagrim
The general view that I have, is that if the Indus Valley Civilization was truly linked, outside basic trade routes, to Sumer and Akkad, the origin was within the sea. It is unlikely in my opinion that there was an actual urban link between Sumer and Elam (not even related to one another), across the Iran desert and Baluch interior. There was not enough humans settled here and the areas are even in the near future lightly populated, even in the Achaemenid period. I do have sympathy with this view though, Europe is an example of a place that I feel has a much deeper Bronze Age than the common low level historian is ready to give. However, Iran is by no means Europe in terms of geographical wealth.
Regardless, my view is that the clear link, if there is one, originated in the Persian gulf. With either, the ancient ports of Dilmun, Magan, etc... or more correctly, the land previously above water in the Persian Gulf. This area gives credence to both Hindu, Sumerian and Elamite flood myths as well as the Sumerians fetish for Dilmun and Eridu. Dilmun, likely now under water, was called the origin for Sumer and Eridu is called the ancient city older than the Deluge.
However, I doubt this fundamental link. If there was such a link, we would find rudiments of or complete cuneiform. Unrelated peoples in Egypt, used cuneiform for political discourse and diplomacy, why then, with a link, do the Indus Valley not use the same? We should be finding cuneiform all over Mohanjo Daro, but we do not find this at all. Either, the sites we are finding in the Indus, are actually irrelevant villages (impossible to imagine) or the link was only one of mercantile origins and no fundamental link existed.
The trade can also be argued, was less than relevant between the two. While Sumerian and Akkadian texts mention the trade; this is not a sure indicator. By this, there is no text mentioning direct transfer, but instead the trade is alluded to have arrived via the ports of their famous allies and vassals in the Gulf, such as Dilmun and Magan. Both of whom, would have been the Middle man between trade. Thus, there was not some grand cultural transfer between the two outside of sharing goods. This aspect is also shown in the lack of fundamental similarities in architecture, not just the writing systems. As far as we know, Indus Valley has no writing system, only proto-writing. It is inconceivable to no find bilingual transcripts or more developed writing (or at least clear influences), if indeed the link were true.