All of these scenarios have unordered conflicting accesses to a single variable (i.e. data races) and therefore have undefined behavior. Scenario A doesn't fail dynamically because Int is a trivial type. However, when reading from a variable like this, the compiler is not required to honor the possibility of a concurrent store, which can cause apparent miscompiles, so it's still not safe.
As other people have mentioned, Swift doesn't directly provide any low-level mechanisms for handling concurrency; you have to use OS facilities like mutexes and Dispatch. We're actively working on improving this; our approach will be centered around making data races statically impossible as much as possible.