Could having cadaver bone be causing my neck to crack frequently?

Doubt it. Cracking has nothing to do with whether the graft is cadaver or not. It may have to do with the way it is positioned in your neck and the way your adjacent structures relate to it, but there is so much chance you would have had cracking had it been a different graft put in.
No. I assume it was for an acdf. If the fusion is solid, the noise is from another level. Noise in and of itself needs no treatment and does not necessarily mean u r destined to have a second fusion.