Rishi Dirghatamas was a great poet but I don't agree with translating 'Devas' as 'Gods' in English. I would rather say 'Devas' are spiritual concepts. So in this sense Deerghatamas's proclamation is true.Absolutely not. Puranic Vishnu is no different from Vedic Vishnu. If Vedic Vishnu was just modeled on Solar noon, we would not have had a hymn like 1.22. Vishnu's trivikrama act in Puranas is description of the same in Rig Veda.
Not just in Puranas, even in Rig Veda Vishnu helps Indra. And no, by no means is Vishnu a minor God in Vedas.
We do not need a better seer than Rishi DIrghatamas who proclaimed that it is the same God with different names.
Concept of Vishnu was definitely modelled after solar noon(a natural entity just like other Vedik Gods) so he represents all- pervasiveness. Purana have established him as a supreme God(like there was a competition between different God) and have turned the Vedik Indra into a caricature.