You say: " Sagan had been considerably more open on the subject of extraterrestrial visitation at the beginning of his career but his public statements on the subject had turned to the negative sometime in the mid-60s".

So I don't know if you are aware of this, but didn't mention it. Early in his career Sagan wrote a Stanford paper in which he openly speculated that the Sumerian civilisation was founded by aliens (think Zecharia Sitchin).

All this is documented and discussed in detail in The Sagan Conspiracy by Donald Zygutis, subtitled 'NASA's Untold Plot to Suppress the People's Scientist's Theory of Ancient Aliens'.

You might also be interested in an article of mine (written before I discovered the above):

https://medium.com/@graham-pemberton/extraordinary-claims-require-extraordinary-evidence-the-ufo-question-and-carl-sagan-24e11ae06e56