It seems that from the results I am getting, and from my limited understanding of the Assimp source code, the aiProcess_MakeLeftHanded does not actually make the model left-handed, but instead it simply reverses the handedness. So if a model that is originally left-handed is loaded with aiProcess_MakeLeftHanded, the model becomes right-handed. Could anyone confirm this?
Yes, that's what the MakeLeftHanded process does. The reason is that there is no way to tell if a mesh or scene is left handed or right handed. You have to guess that from the context (i.e. the source file format) and work from there.