Q: What about the signs where you have some very convincing thoughts, the trader and so forth. The logical basis that traders would have to be represented in some way on seals which were meant for trade makes sense. [See "Dictionary" for Mahadevan's speculations]
A: It is a moot question whether the Harappans had castes like we have in later India. But they certainly must have had occupational groups, priests, scribes, traders, warriors, why not? Perhaps they were not very rigid divisions. In any case, even if these terminal ideograms represent different groups, we must remember that the ideograms themselves are combined - for example the symbol of a bearer is combined with that of a jar, and that is followed by a harrow sometimes. So they could not have referred to exclusive caste groups like we understand the term. The possibility is that originally these were all mere phonetic symbols, but later the groups whose names used these symbols most, they only had surviving symbols with the loss of the language, so the symbols became mythical symbols or representations of their own cult objects. It is a jungle really, very difficult to trace out the environment in India which is not only multi-cultural, multi-racial and multi-linguistic.
The Dravidians and the Indo-Aryans interacted with each other and we have the Indo-Aryan languages with Dravidian features and we have Dravidian languages with Indo-Aryan features. Hinduism has both Aryan and Dravidian elements, not to speak of modern Hinduism also being influenced by Islam and its own native offshoots like Buddhism and Jainism. There are no clear cut parallels.
What is more, the same symbol in the Indus valley could have taken multiple forms later. The jar sign is one example. Kunda is both jar and the fire pit. In later India, there are communities which claim to have arisen from the jar, and other from the fire pit. Now both have the same word and both could be two very different responses to the same common tradition.