I had a partial case head separation using a 1.8oz buffer

[Klem]

If the bullet slid forward and was into the lands during chambering, could that have created enough pressure to blow the casing like that?

Extremely unlikely - bullets jammed in the lands raise the pressure/velocity, but nowhere near blowing out the base of a case. Some bench-rest shooters deliberately load their rounds long so they jam into the lands as the bolt is closed.