I am under the impression that to review frrom mobile one can only go to the full site theme and review from it. Am I right? If so, is there any reason why there is no way of doing this from the app or mobile site? And if I'm wrong and there is such a way, which is it?
Those features are not available in the app (iOS and Android) because the Stack API version 2.2 doesn't have endpoints for that.
The mobile apps use the API to do the things they need to do. So before they can even start implementing that on those native platforms, they also need to have a 2.3 or maybe even 3.0 version of the Stack API available that does expose a /review endpoint.
Given the nature of reviewing I don't expect this to be high on the feature list though but that is just speculation.