I think you'll be able to straighten it up by being clever in how you stable them. I recognize this from building block towers. Start from the bottom, then line up and glue the next piece onto the one below and repeat with the next one, while keeping things lined up all the way up. Only use a few drops of glue, so it will be possibly to pry the pieces apart again after molding if you want them to remain separate - or just glue them solid if you have no use for an individual parts vertebrae afterward.
Then, you simply and very delicately start to fill the undercuts between the pieces, while keeping the illusion that the separations between them are deep - the filling is only to ease molding, so the piece is a "solid" piece being molded and not with gabs between the pieces.
Hope this makes sense. But this was how I was planning on doing this, while searching for a proper looking vertebrae that wasn't too expensive, but I've not gotten beyond the planning stage so far.
Good luck.