Boris Gudenuf
Deity
SInce the concepts of Indian Caste System came with the Vedic conquest. I might say that Harappan caste systme was more or less different to Vedic Era. Particularly Ramayana emphasis on the Vedic superiority and emphasis on Warrior (Ksatriya) caste and to back Monarchy system. However there were Commonwealth style government exists in Old India and Ksatriya as Old Indians (maybe Harrappans too!) know were very different to those of Khmer and their successor states (particularly Ayutthaya which later formed what's known as Siam to the World much later on). While those of Khmers and Siamese assigned meanings of the term Ksatriya and Raj as one and same (And means King), Indians as i've just know in very recent years, referred to this caste as something akin to Euro Knights or Japanese Samurais, and the 'Raj' was more or less different to Ksatriyas.
As far as I know, there is no evidence for a Caste System in the Indus Valley Civilization. In fact, since there are no foundations of 'Palace Complexes", there is not a lot of evidence for the sort of Extreme Stratification you get in Mesopotamia, the Mediterranean, Chinese, and later Indian states in which a tiny group is fabulously rich and the majority are either dirt poor or nearly so. Financial/physical 'egality' does not necessarily mean that there wasn't extreme social/cultural inequality, but we simply don't know.
I'd say Harappans might be a kinda confederacy rather than a single Empire. In India, the concepts of 'Emperor' (Chakrabhadi: means a person whom his chariot wheel rolls undeterred) was so grand that only a few king / president may earn this title.
Also did Varanasi (กรุงพาราณสี) associated to Harappans or Vedics? there were many siamese folklore citing of this city under their renditions (Krung Pharanasi) ruled by a great 'king' maned 'Promatat' (Bhrama Dati, The Gifted One) and peasantry with local Thai names. Did you know the identity of this mythical king who he was? and which era did he associated to? Harappans or Vedic or later?
I tend to agree with you on the concept of the Indus Civilization as more of a city state confederacy than a single political totality/empire.
But here's the crux of the problem, and also to the answer to any questions about Vedics and their relationship to the earlier Indus culture - we literally Do Not Have A Clue. We can't read any of the Indus Valley script, we don't even know what language family it relates to. We do not know if the "Aryan"/Indo-European/Vedic" peoples arrived in the area with nothing but chariots and picked up all their other cultural traits from the Indus Valley people, or they completely wiped out all cultural traces of the earlier Civ and replaced them.
Both of those two possibilities, by the way, are the least likely. Everything we know about 'cultural fusion' tells us that the most likely scenario is that the Vedic Culture was a combination of traits they brought with them, traits they picked up from the existing people (who, after all, being part of a wide-spread civil population, probably outnumbered them by a wide margin) and traits that they would insist are still original but in fact have changed beyond all recognition.
Because it shows up in virtually every Human culture, from Mesopotamia to China to Mongolia to Rome, Greece, North America, Central America, etc. we can be sure the Harappans had a 'Founder' myth of some kind, but, again, and I'll keep repeating it: We Do Not Know. Of the thousands of figurines found in Indus archeological sites, we cannot even say which of them are Religious (if any) and what, if they are, any of them mean. There are a lot of models of cattle/oxen, and the Indus may have been one of the first civilizations to domesticate cattle as draft animals - and therefore, like later Mari in Mesopotamia, make them part of their financial and belief system, because they were so important - but (repeat after me) We Do Not Know. It would be really nice if we found an unmistakable Altar in a Harappan site with a set of Horns of Divinity on it, as we find all over the Middle East and Crete, which would solidify Cattle as important to their religion, but right now, for all we know, they worshipped Terry Pratchett's Great Om