From what/where is the title "queen" derived from, in an etymological sense?

Well, you were already on the right lines with your previous research into king. The queen originally was just the king's lady (rather than a ruler in her own right), at least with Anglo-Saxons. As far as I can see the word goes back through Old English cwen (pronounced like queen), Gothic qens, Old Norse kv�n or kvan, right back to the Greek Gyne, meaning woman or womb (good old Chambers).