Re: split cases
The case splitting definately sounds like a headspace issue. It would be intersting to take a headspace measurement on the datum line of a new case and then compare it with a once fired. I had a Rem 700 that would stretch the shoulders .014 on fireforming but it was still able to get 5 or more reloads, depending on the brass. Good chance both these barrels were chambered with the same reamer.
The flattened primers sound more of a pressure problem. Maybe try backing the load down some or seating a little deeper. Or both.