IMO the crux of the problem with the Phoenix in "First Contact" is that they insisted that Cochrane developed the warp drive basically in his garage, in the middle of a new dark age. Right.
That was the problem I had too. In the Reeves-Stevens novel "Federation," it was established that Cochrane worked with a whole team of scientists to develop warp drive, and it was only after he had completed his prototype that the war started. That makes much more sense than what was shown in the movie. However, Star Trek seems to have always had a problem with technology being developed by committee; take the transporter in ENT. Again, they establish that it was created by just one guy.
Has it ever actually been established that the ring on the XCV-330 is in fact its warp drive--assuming it was a warp ship at all? I've always has the impression that it was a habitation ring, which rotated to create artificial gravity
No, it was never established to be a warp drive. It was only after the Vulcan ships in ENT were shown that people equated their design with this old drawing. IIRC, Doug Drexler said he was influenced by the design, but that doesn't mean that the ship had annular warp drive as well. As a matter of fact, the drawing seems to show the propulsion as rocket exhaust coming from the rear of the ship.