If it wasn't for the spines, a plaster mold would work (properly sealed of course.) Perhaps you could make a stack mold (essentially a series of 2-part molds) that would allow you to pour the candle in pieces, then assemble it as a stack, with a hole for the wick drilled out as you go (so the wick would have the curve and stay centered the entire length.) If you incorporate a hole and plug 'socket' feature in the pieces, then it will be much sturdier during assembly. The fact it is a spine sculpture helps since visible 'joints' are desired.
In short, mold the vertebrae individually and assemble like a real spine!
Regards, Robert