and they lack any Jenova, as shown in their lack of a true reunion instinct
That doesn't make a whole lot of sense. They are the manifestation of Sephiroth's spiritual leftovers, thus they are comprised of his essence - a fusion of Jenova and human aspects. They most certainly had a reunion instinct, as shown by their gripping desire to acquire Jenova's cells and their ever-present awareness and need to bring Sephiroth back into the world.

Jenova was an immensely powerful being that survived travel among the starts and the fall to ground; she almost eradicated the Cetra, and then survived two millennia sealed in rock. Even tiny severed portions of her body can re-form into dangerous monsters, as with Jenova-BIRTH, LIFE and DEATH. Every clue, both in the game and the movie, points toward Jenova cells being the source of some dangerously potent abilities.

As for Vincent... what does the Ultimania say about him, particularly the experiments performed on him after Hojo killed him? I'm curious about the details.