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I haven't fully grokked your code yet, but the bolded question has a straightforward answer:

Can I always rely on RMW operations not being reordered - even if they affect different memory locations

No, you can't. Compile-time reordering of two relaxed RMWs in the same thread is very much allowed. (I think runtime reordering of two RMWs is probably impossible in practice on most CPUs. ISO C++ doesn't distinguish compile-time vs. run-time for this.)

But note that an atomic RMW includes both a load and a store, and both parts have to stay together. So any kind of RMW can't move earlier past an acquire operation, or later past a release operation.

Also, of course the RMW itself being a release and/or acquire operation can stop reordering in one or the other direction.

Of course, the C++ memory model isn't formally defined in terms of local reordering of access to cache-coherent shared memory, only in terms of synchronizing with another thread and creating a happens-before / after relationship. But if you ignore IRIW reordering (2 reader threads not agreeing on the order of two writer threads doing independent stores to different variables) it's pretty much 2 different ways to model the same thing.