Did Buddha Achieve Enlightenment?

I say "No." Because that way of expressing things gets it backwards. From the point of view of the possesive ego, enlightenment is a disaster: it confirms that ego's worst fear: that it does not really exist as a substantial entity. From the point of view of Siddhartha Gautama's pre-enlightened ego, he completely screwed up at that moment he stopped trying to achieve enlightenment (which all along was a trick of the ego to block actually achieving it) and accidentally did so in the process.

This is what Tolkein called the "eucatastrophe," which is portrayed symbolically at the climax of The Lord of the Rings. The ego (Frodo) can tell itself it is on a quest to destroy the ring of power (its attempt to manipulate reality to sustain the illusion of its own solidity), but it cannot achieve its goal: its very belief in its existence as a solid entity depends upon the failure of the quest, and at the edge of destruction (as it sees things), it attempts to seize the ring for itself.

But undertaking the quest was not pointless: by undertaking it, the ego has dragged the demonic forces keeping it in bondage (represented by Gollum) to the edge of destruction as well. They seize the ring and fall into the fires of Mount Doom.

Now what was previously understood to be destruction is correctly understood as liberation: there never was any solid entity to be destroyed in the first place, and the illusion that there was (samsara, or bondage to sin in Christian terms) had never been the only hope for safety, as it had appeared to be, but a prison.