I agree with Ginnie that a second opinion (at least) is in order, not only for whether surgery is necessary, but also if the proposed procedures are the only ones appropriate, or if there are other alternatives—surgical or otherwise.

Cadaver bone may be less painful, but autogenous bone (originating from the patient's own body) is usually preferred because there is less risk of rejection.

Mixing may have to do with reducing/limiting rejection risk while also minimizing the amount needed to be taken from your own body. Just my guess.

Doc