My Wyandottes, who are very good layers when they're not broody, go through a cup of oyster shell a week on average, even when they are on layer feed. There are many variables. No pet chickens here after my 1st batch 29 years ago.
Setting aside, for a moment, that this is mere anecdote - I'm not sure what you are trying to prove here?
Is it that excess calcium isn't a problem for infrequent layers of medium eggs because your Wyans are "very good layers"? Nor, absent a flock count, does a "cup of oyster shell a week" give us any idea of how many birds are eating it. While "even when they are on layer feed" suggests that there are times when they are not. Does "no pet chickens here" mean you are culling/rehoming after the first year or two of production, as commercial or home-based limited commercial egg producer might do? Or are you keeping them till they fall over, then opening them up and poking around inside for cause?
In short, your posts have been long on supposition, but you have yet to cite anything in support of your claims which has even the illusion of scientific rigor. Your questions, however, are (thus far) easily answered.
If, as you propose, "
Feather Fixer" is deliberately crafted for molting birds, a period you suggest is 4-6 weeks, why is it sold in 50# bags, suitable for feeding six birds for more than a month? Since they don't all molt at the same time, wouldn't a smaller package be more appropriate, similar to the way Starter/Grower is usually marketed?
As to what they know, their "Layer" feed in the same product line
is here. The difference between the two? FF has 2% more protein, 1.5% more fat, and 0.3% less calcium. Disclosed amino acid content is unchanged. Which sort of of begs the question why,
if feathers are primarily keratins crosslinked by disulfide bonds made from the sulphur-containing amino acids (Methionine, cysteine, homocysteine, and taurine), and only about 1.2% fat, does Feather Fixer have more fat than Layer feed but no adjustment to amino acid levels?
At this point, I'm inclined to believe Nutrena is more motivated by marketing than Science in purveying "FeatherFixer" to the backyard owner - though compared to their layer feed, it is a superior offering. Their All Flock, with free-choice oyster,
is better still.
Nutrena, in fact, knows so much about poultry that they helpfully write this, "May also be fed to laying hens and mature roosters year-round for optimal feathering and brilliant plumage" on their Feather Fixer page (linked above) and follow it with this - "Make oyster shell available free choice from the onset of lay". Good to know they think Roosters are appropriately fed on a diet of 3.5% calcium all their adult lives...