Has anyone ever one a "pure" diplomatic victory where they did not have to conquer (or have conquered) or absorb any other races?
Yes; recently, too, playing DA 1.50x (before the most recent patch and fix). I was playing a neutral-aligned custom race with the Super Diplomat ability. There were six remaining AI factions in a large galaxy, of all three alignments. It was fairly far into the game, but I hadn't been at war with anyone yet.
I was about to mix it up with the Krynn when I saved and quit for the night. After reloading the save the game crashed every time I hit the end turn button. This happened every time, no matter how I played the turn.
Eventually I noticed that I was 'close' with all of the AI factions, so I tried for a blitz alliance victory. I managed to get alliances with everyone, though I had to give the Thalans Black Hole Projectors to secure theirs. This time when I pressed end turn, I got the victory screen. Not a bad score considering I only had twelve planets, so I just posted it while I could.