So you have extensively given references from a specific interpretation of "vyuhas" according to Pāñcarātra theology in your question.
But since you have not asked specifically for an explanation from the point of view of Pāñcarātra, I will answer from the point of view of generally accepted Vedic tradition, as seen in Sāyaṇācārya's commentary.
Is it possible for the Vedas to have been authored?
Yes, they were authored. First off, here is some background information: If a text is authored, then it can be doubted because the author might be wrong. Thus we can doubt the Bible, Koran, etc. and all other spiritual and scientific texts because the writers could be wrong. But if a text is authorless, ...
The seer (rishi) is Visvedevas (Viśvedevāḥ) according to this Krishna-yajurveda Taittiriya Samhita published by Svadhyaya Mandal, Pune in 1957.
The screenshot below is from the Kanda 4, Prapathaka 4, Anuvaka 12. The Nila Sukta starts with gṛṇāhi | ghṛtavatī. Viśvedevāḥ is the seer for entire 4.4.12.