I believe, what you found in the seat is flame cutting from the primer. I found that the CVA/BPI breech plugs would start to develop the grooves some ware less than 200 primers. At one time I believe CVA/BPI recommended that the plugs be replaced every 400 primers. In working with CVA, Remington, Knight, Lyman/Pedersolie, and my own from Grade 8 bolts. the CVA were the softest most easily machinable material. This might be why the erosion starts so soon.

The o-ring might be a good plan for the plugs to help stop or slow down the starting of the erosion. The plug you have now that shows the erosion could have it's primer pocket deepened and that would clean up the erosion and allow space for the o-ring. this would definitely lengthen the service life of the plug. If the plug then developed more erosion, you could replace the primer seat by drilling and taping the plug for a short length of 3/8x24 grad 8 bolt, then make a new primer seat in the bolt with a C size drill. I would recommend purchasing several C size drills from different makes, this will allow you to fine tune the actual diameter of the hole for your primers. I have 3 different C size drills and they all differ slightly in size, something about with in normal limits +/- . When doing this I run the bolt clean threw the plug so the vent liner is fitted in it also.