You are right, I think the damage was progressive and only became worse. After close inspection, I did cut the channel too deep and removed much of the strength in the direction of the channel. I have converted a steel 1851 and left much more metal and I would be very careful about the amount of metal removed from the next brass model.
I understand the manufacture's stand. A steel pistol would not have suffered the same consequences under the same conditions. At the price of the pistol, I think is worth trying again at least one more time.
Taking a file or Dremel to your pistol is not for the faint of heart and should not be undertaken lightly. Sometimes a few mistakes need to be made and holding that beautiful converted brass frame 1851 Navy Sheriff is worth trying again for me.