Thanks for your insights. (2) makes sense to me, but for (1), on ARM64 can an aligned 64-bit store really tear in a 64-bit non-atomic load? The spec says "A write that is generated by a store instruction that stores a single general-purpose register and is aligned to the
size of the write in the instruction is single-copy atomic" (B2.2.1)
Well, if you target a specific architecture, then of course you can assume more guarantees than in general, portable code. And in general, a processor might distinguish between non-atomic and relaxed-atomic reads and writes – in theory.
But more important, and relevant in practice, is the behavior of the compiler. C, C++, and Rust compilers are allowed to assume that non-atomic reads aren't influenced by concurrent writes, so the compiler is allowed to split non-atomic reads into smaller reads (unlikely) or even optimize the reads away if it can prove that the memory location isn't written to by the local thread (more likely).
