Word order conventions are learnable. All natural languages can say anything any other language can say. So don't mix chimps and humans. And creole has full-blown UG, not part of it. Some grammatical rules are needed to use strings of symbols as propositions that can be true or false and that can be used to get and give the information we convey with sentences.
Processing becomes quicker and more efficient.
A concept cannot be true or false. It is not enough to assert that something must have had an adaptive advantage just because it's there. You have to give some idea of what that advantage might have been. To explain UG as an evolutionary adaptation is not difficult. Remember to focus on UG, not language as a whole.