Oxalates bind to calcium and prevent it from being absorbed and it's excremented from their chloeca. Unless that that changed in biochemistry, it's not a misconception.You have a few of the common misconceptions that circulate around in the tortoise world, and I'd like to help you catch up to speed with the members of this forum. We've learned a lot in the last decade or so. Online sources, government sources, vets, and CTTC are some of the worst offenders with terrible care info, and they don't want to hear it when people tell them otherwise. They fancy themselves "experts" and ignore the three decades of findings from someone who has raised dozens of DT hatchlings in myriad ways, as well as hundreds of hatchlings of other species, and done side-by-side experiments, in a captive environment (which is what we are all talking about), and they instead spout weather and climate statistics that have little to do with an animal living primarily underground.
First it is wonderful that you rescued these animals. I cannot understand why you would be against breeding an endangered species that makes a wonderful tortoise pet, but I do know that it is illegal. That is a terrible shame in my opinion, but that is a discussion for another time that I'd be happy to have.
Most of the care info given for this species is all wrong, especially when applied to babies. Following this old wrong info that is based on misconceptions of how they live in the wild, often results in the death of the animals. With this in mind, I'll lay some info on you:
-The diet items you recommend are excellent. Top notch and I use and grow most of the same ones myself. I prefer fresh grown alfalfa, in small amounts once in a while to dry alfalfa hay, but other than that I say thumbs up to all that you mentioned. I'd add the there are some darn good ways to supplement their diets now too. And "hell yes" to lots of opuntia pads!
-I don't know why you have fixated on oxalates, but oxalates are not the boogeyman that we once thought they were. Tortoises handle and process oxalates, and many other compounds, differently than mammals do. Oxalates do not cause pyramiding or bladder stones. Both of those are caused by dryness and dehydration.
-Fruit: I say: "If it is so bad that you can only feed a small amount once in a while, why feed it at all?" Instead I recommend people offer hibiscus flowers, dandelions or opuntia "fruits" as treats.
-Pyramiding is CAUSED by growth in conditions that are too dry. Not by food, not by too much protein, not by lack of calcium, not by neglect, and not by too small of an enclosure. All of these things are bad, but they don't cause pyramiding.
-The biggest killers of these tortoises are dehydration and dogs, in that order. Yes they are desert animals, but the are masters at avoiding the extremes by living a fossorial lifestyle. When we house them above ground in our yards, they are exposed to temperature and humidity extremes that they would not experience in the wild. They need to be soaked regularly.
-Outside all day is NOT good for baby DTs, contrary to popular opinion. Some limited sunshine for babies and little ones is good, but "housing them outside" is NOT better for them or good. Adults yes. Babies no. A baby in the wild would remain hidden and tucked in to tight cover most of every day. If it didn't, it would be eaten by a raven or coyote very quickly. In our captive enclosures, they feel safe and walk around out in the open all day, exposing themselves to conditions that would seldom be endured in the wild.
Questions and arguments are welcome. Feel free to ask who the heck I am to say such things, and make me explain why these assertions are correct and all the other assertions you've read over the years are wrong. We are all here to talk torts!