> I ask a simplest of questions about it, one that any bright primary schooler will come up - and the understanding immediately implodes on itself because the answer is simply not there.No, the answer is there, but it's not simple. Again, you want a quick sound bite: yes or no to your question. And reality is not like that. Reality is not simple sound bites. Reality is complicated and it takes work to figure out what is really going on. That's why it takes years or decades to get a real understanding of any useful model of reality on the scale of the universe. That's why it takes years or decades to do research to expand the boundaries of our knowledge. You want me to just bypass all that and give you a simple answer. Sorry, no. There isn't one. That's not because scientists are stupid or because our models don't work. It's because you don't want to accept the actual cost of understanding them.
> Imagine how a person will study mechanical springs for several decades
But nobody does that. Physics students learn a simple model of how mechanical springs work in a lecture or two. Mechanical engineers learn more detailed models that work for things like designing car suspensions in maybe a semester. People might certainly work with mechanical springs for decades, but they won't spend that whole time just getting an understanding of our best current model of them. They will spend most of that time using the model that that already understand.
But the whole universe is not a mechanical spring. And learning our best current model of the whole universe is a lot harder than learning our best current model of a mechanical spring. So you are simply being unrealistic if you expect someone to explain the whole universe to you with the same amount of effort it would take to explain a mechanical spring.
> I understand the idea of local conservation.
No, you don't. You may think you do, but if you did, you would not even have to ask:
> Imagine an empty region of space where there's just light, and neutrinos, and some dust. Some time passed and some of that light (and neutrinos) lost energy due to red shift. Where did that energy went, stress-tensor or not?
Local conservation says no stress-energy went anywhere. The redshift due to the universe's expansion does not lose any stress-energy. Stress-energy is conserved.
A better question would be, if stress-energy is locally conserved, why is there a redshift as the universe expands? And the answer to that is that the local conservation law does not say the energy density stays the same. It says that stress-energy is not created or destroyed. But "stress-energy" is not the same as energy density. The density of stress-energy, for matter (or radiation, since you asked about light) is the same with the scale factor slightly smaller and the energy density slightly higher, as with the scale factor slightly larger and the energy density slightly smaller. That's what the equations tell us. (For dark energy, as I've said, they tell us that the energy density does stay the same as the universe expands. But that's only true for dark energy.) All of these statements are consequences of the Friedmann equations, or, if you want to look at it that way, of the relationship that is enforced by those equations between stress-energy and spacetime geometry.
That's about the best I can do at describing what the equations say in ordinary language. There's a reason why physicists do physics with math: because math is a much better tool for the job. I've already told you what math to look up if you want more information.