The mechanical knowledge and detail analysis you folks post is remarkable. Yet I wanted to respond because years ago I loaded for the 7mm Rem Mag and did not have such issues with difficult extraction. Seems I recall that even though the 7mm Rem Mag has a belted case it still headspaces off the shoulder, not the belt. Using once fired cases I always neck sized after fire forming which should eliminate any headspace issue for that particular case in that particular rifle. That said, if I continued to experience pressure issues with well established loads I would be concerned about mechanical issues with the action, locking lugs engagement, etc. regarding setback and/or undersize bore, heavily fouled barrel, etc. If the loads are good something is wrong with the gun and it needs inspection by a competent gunsmith. Clearly from the primer craters and more this gun has experienced high pressures and those may induce some of the mechanical issues with the action that may not have existed before the excessive pressure rounds had been fired in the gun.
My two cents,