My (very limited) understanding, mostly from the book "In the Dojo" by Dave Lowry, is that uke is from the verb ukeru, meaning to receive (Uke is one who receives) and nage is from the verb nageru, to throw (Nage is the one throws).
Yes, basically. (As an aside, I am a bit confounded when people "translate" the word "ukemi" to "the art of falling" or apply the term "ukemi" to falls, as the word really doesn't seem to have any connotation of "falling.")
A completely wild guess on my part is that there must be a verb like toriru (?) meaning to grab, and Tori would be the one who grabs. Just a slightly different word for the same idea as Nage.
The infinitive verb that the noun "tori" is derived from is "toru" which basically means "to take."
To me, there is a subtle but interesting difference in the terms "nage" and "tori" when applied to the role opposite of uke. "Nage" to me has too much of an emphasis on "throwing" whereas the term "tori" connotes to me more of a sense of "taking" what is actually sensical and available. The same kind of nuance exists in the Yoshinkan term for that role of "shite" which, to me, basically means "the doer" or "the protagonist" (from the use of that term in Noh theatre). I prefer the semantic nuances underlying "tori" and "shite" moreso than "nage," personally.
Just my thoughts,