I'm really curious if it's usage was political. I did find this forum post[0], but it didn't really shed any light on the question. It does, however, suggest that the term was chosen in ignorance of it already existing in fiction for (at least) decades by the time the space race had started. So, if the forum post is to be believed (I couldn't find the cited first source), a defense suggesting it was already in the lexicon couldn't be supported since the selectors of the term themselves admit their ignorance of the term previously being used.
Cosmonaut, I believe, more literally translates to "spacenaut", which sounds terrible, because of the Russian word for space, which, like our cosmos, is derived from the word kosmos. The English term cosmonaut is not through the greek kosmos and nautis, but just an Anglicization of the Russian word.
[0] - http://www.collectspace.com/ubb/Forum34/HTML/000108.html