Marcion, are you arguing for the idea that the Vedas were somehow merging primitive polytheism with mystical monotheism? I have been asking Hindus why so many Hindus require the multiple deity system if all their orientation is to the single Supreme God whether named Brahma, Shiva, Vishnu, Krishna, etc.
An Ista Deva is something different from the God who is the absolute reality which is reached by people who become enlightened.
God can be described as the Supreme Consciousness, the 'Parama Purusha'.
The Ista Deva is considered God (Narayana) because He has a whole list of supernatural and some more mundane qualities, but He is not the same as the unmanifested God because He has taken on a certain shape and personality within time and space.
The Ista Deva is a bridge between God as the unexpressed Supreme Consciousness (Nirguna Brahma) and God as the creator of the universe (Saguna Brahma) and can do so at different times and in different shapes according to the needs of the times.
This relationship with an Ista Deva is derived from the need for a personal relationship with a great master who serves as both an instructor and a figure through Whom the disciple can direct their love for God. Perhaps this is comparable to Jesus and the Father/Abba where Jesus is also seen as more or less spiritually merged with his Father and serves a similar role to the Ista Deva in hinduism.
Of course there are also hindus who will say that this or that Ista Deva of others is only a "half-god" or perhaps even just a mythical creation. That does not surprise me because how can you know the actual historical reality of such ancient persona who have been worshipped for thousands of years? We don't even have proof that Jesus was a historical person or what his real status was.
I think it is only the Smarta part of hinduism that actually worships multiple Ista Deva's, most hindus stick to just one just like christians do.
There is no question of an actual mixing of polytheism and monotheism, it is more that the old polytheism that was connected to forces in nature disappeared while being replaced by more sophisticated explanations of a single God with His different aspects. A lot can be said about God and hindus give these aspects different names, such as Brahma, Vishnu and Mahesvara (also Shiva) for the creative, the operative and the destructive parts of God within His manifested universe.
You see how complicated things get when they start mixing up these names for aspects of God with an Ista Deva who actually lived as a historical spiritual master.
Shiva was such a master who many accept as their Ista Deva but in the very complex hindu mythology Shiva has also become the aspect of God that destroys the things He created and operated earlier.
This problem of all kinds of older and newer layers in hinduism can also be found in christianity when you pick apart the developmental layers of the New Testament. For practising hindus and christians this is not a problem because they will understand their religion and its scriptures as a unified whole through the coloured lense of their own religious sect and explain everything accordingly, so in fact through the lense of one the later layers.
In the sect where I learnt to meditate they strip away many of these layers and all of the mythology, a bit like what happened in Buddhism where only Nirvana (Saguna Brahma) and Mahanirvana (Nirgina Brahma) remained.