That's a possibility. Some code still assumes (without realizing!) x86 style ordered loads and stores. This is called a strong memory model, specifically TSO, Total Store Order. If you tell x86 to execute "a=1; b=2;", it will always store value to 'a' first. Of course compilers might reorder stores and loads, but that's another matter.
ARM is free to reorder stores and loads. This is called a weak memory model. So unless it's explicitly told to the compiler, like C++ memory_order::acquire and memory_order::release, you might get invalid behavior. Heisenbugs in the worst case.
Depends on the processor's store queue. It might be. Of course if otherMethod() is anything more complicated, 'a' will almost certainly be assigned first. Of course you should still do the order dependent assignment correctly, like by using C++ std::memory_order or atomics.
ARM is free to reorder stores and loads. This is called a weak memory model. So unless it's explicitly told to the compiler, like C++ memory_order::acquire and memory_order::release, you might get invalid behavior. Heisenbugs in the worst case.