In fact, with Memory ending up as a null value, StructureSpawner.createCreep() causes a TypeError: Cannot read property 'creeps' of null, because it assumes that Memory can only be undefined.
Specifically: p.isUndefined(k.Memory.creeps) && (k.Memory.creeps = {}),
http://support.screeps.com/hc/en-us/articles/205619121-RawMemory
http://support.screeps.com/hc/en-us/articles/203016642-Working-with-Memory
Assignment to the Memory global doesn't persist outside of the function that it's assigned to, unlike what is described in the second article.
Working with RawMemory seems to result in Memory being set to null, and then all of the "helper" memory attributes failing because they're looking for a memory global that no longer exists.
Would it be possible for a method to be exposed to change the global Memory object? eg. RawMemory.setMemoryGlobal(obj)