You can't detect damage? I can--possible damage or blemishes, of course, given that I don't have access to the original papyrus. Do you?
Look just above the top "blotch" on Metcalfe's Figure 2. There is an apparent blemish or "white out" that runs from left to right, angling ever so slightly down and to the right. It appears to cut across the bottom of the top three fingers. In Rhodes's photos, the blemish appears even more pronounced and "white."
Now look at Metcalfe's Figure 4, the real bird. Note a similar blemish or "white out" in the third line from the left on the bird's left wing (the three lines represent downward feathers). Obviously, that line should have run uninterrupted, but no, there's a blemish of some sort at about the middle of the line. At least, there is no ink where there should be. The blemish even cuts into the second line from the left just a little.
Likewise, the blemish or "white out" on the finger stroke, creates at least one of the so-called "blotches" Metcalfe wants us to believe are feathers.
In fact, the more I look at the lion couch scene, the more such blemishes I discover, i.e., there is no ink where there should be ink. Take a look at Abraham's kilt. Blemishes or "white outs" in many of the lines that decorate the kilt. Look at the first layer of what the Prophet calls the "expanse," the herring-bone floor that the couch sits on. Once again, a number of blemishes or "white outs" in the lines where there should be ink. Likewise in the pillars of heaven. I guess we could call the resulting "blotches" bird feathers, but I choose to reserve that term for Metcalfe's argument.
At least that's what I see.
What you are describing is not damage, but rather, the texture of the papyrus. The pen/brush sometimes skips across low-lying grooves, which appear white, but this can be sometimes be deceiving. For example, it appears here that the line extending from the middle left to the upper right has been cut by one of your "white outs". However, each line segment actually belongs to a different character. Also, with sufficient ink flow, the "blemish" is filled in, as seen in the center character.
Since the recto and verso fibers run perpendicular to each other, real damage changes the underlying texture.
You'd be better off arguing that the scribe's stroke was too rapid for the ink to fill the groove.