Every language learner goes through steps in acquiring language skills that are marked by certain milestones. The rate of progress varies, but the order of the steps has been found to be consistent.
People who use AAC need the same opportunities to communicate at each language stage as typically developing speakers. If we approach teaching language in small orderly steps or stages, the AAC speakers have better opportunities to learn.
It is best to let the person try the device - hands on, or any other type of access. The Unity versions with more keys provide larger vocabularies. Versions with larger keys have smaller vocabularies. The one-hit versions have small vocabularies with immediate feedback; you hit a key and immediately hear voice output. The sequenced versions usually require two or three key hits before you hear voice output, but the amount of vocabulary is dramatically increased.
Unity began with just one version that had a large vocabulary and a small key size. Later versions were added with larger keys and smaller vocabulary sets to accommodate access, vision, and cognition issues.
No, most beginning language learners learn Unity by learning a motor pattern, not by learning grammar rules.
No, the metaphors associated with Unity icons are more useful to adults who are helping. Some students will benefit from learning the metaphors.
No. People who are learning to communicate use a variety of modes for communication, including facial expression, changes in muscle tone, gestures, vocalisation, and word approximations. Adding augmentative communication simply adds one more technique to the modes that already exist and only enhances the communication that a person is already using.