Sorry HF, I realise you have been doing a lot of research to try to prove me wrong but your answer is far to long for me to read in detail but I'll have a skim through it.
I agree that layers formlations may contain all that young birds up to commercial slaughter age will require if fed only on the formulation. My birds are older hybrids 4 and 6 years and unlike less well fed hens are still laying 4-5 excellent strong shelled eggs per week each.
Although living in Kent which some may thing a chalky soil my soil it is, in fact, from the greensand ridge and is poor in calcium. My hens have removed all the snails from the garden - which as you know are good at concentrating calcium in their shells - so I doubt that my hens get much calcium from a "wild" diet. They get a little from the grass I would think as all green veg. contains a small amount of calcium (depending on the soil content).
I do understand the physiology of calcium uptake and utilisation as I studied physiology for two years at college. I do understand the relationship between calcium, phosphorous, Vit D, and hormonal control and that only 20% of the calcium ingested is taken up in the gut the rest is passed out. If hens are eating too much meat, e.g. worms, they can have a phosphorous imbalance (too much) which will reduce uptake of calcium too. Magnesium, copper, boron, zinc, manganese and even strontium all have a part to play in bone health; and as you point out the older a hen gets the more health and egg laying problems can build up if they are not getting the correct balance of nutrients.
I try to keep my advice simple and easy to understand for most hen keepers without a degree in a biological science to follow, so:
If your hens free range a lot of the time offer them extra calcium in the form of oyster shell. If they are laying thin shelled eggs give them some extra shell building nutrition. If they are getting on in years be careful to give them a few supliments as and when you judge needed. There is not a one size fits all for hens or any animals for that matter.