It is not possible to rework the bulkhead, nor debur and polish, as that bulkhead is up against a doubler angle on the back, removal of which is not practical. Any attempt at rework would jeopardize the angle behind, which is critical structure. A larger radii would not hurt, if formed before assembly, but the structural characteristics of the bulkhead and angle are very different, so they stretch differently with airframe loads. The angle is more carrying the tensile load between the struts, and the bulkhead is more trying to maintain the stiffness of the doorpost portion of the fuselage (doorposts displacing as a parallelogram under load). The bulkhead is not up to the task in some cases.
The service letter does not mention NDI inspection processes, so I infer it's a visual inspection. NDI would be pretty hard to do effectively down in there.
For pilots, an indicator that the fuselage has a bit too much "give" in the axis of concern is that when the airplane is taxied over uneven ground, the base of the windshield moves left/right relative to the glaresheild at the base of the windshield. For my experience, a 1/8" lateral movement is common for an airworthy fuselage while taxiing slowly over undulating ground, movement of 1/2" laterally would have me inspecting the plane before further flight. But, that's my subjective criteria, by no means authoritative, nor "quantified", as undulating ground is not quantified.