A couple of thoughts here.
As long as that flat does not have extruded metal at the edges that cutout will not hurt a thing. It is on the pressure side of the oring gland so any extrusion will happen on the opposite side of the piston. Run your finger over the ends of the groove and if does not catch, leave it alone. Since that groove will not hurt anything and more than one has had them, it may be some form of indexing or retaining notch for one of the machining processes....or it was stamped out of the edge of the material but still met QC requirements...no clue but in any case it will not impact the performance of the piston, on the other side would be a totally different matter. .
As for the knife end, Couv correct me if I am out of line here but a few psi of creep after lock up that stops is not a big deal, messing with the knife edge is asking to make it worse, that is more of a last ditch effort to restore an edge with an obvious defect that continues to leak past causing IP to keep going up. 145 is not an excessive IP, out of spec some maybe but still better than destroying a working piston over. Would it not make sense to adjust HP seat height to lower the IP, assuming there are no shims that have been missed rather than take the chance of damaging what appears to be working knife edge?