#2, Nope, you can only use the keywords that power bi Q&A defined. I don't think undefined or customize keywords can work with the Q&A feature. If you mean to use a mobile device voice helper to open power bi mobile app, it is possible.
Even in IOS it seems Q&A functionality is not fully available. I am use this option on my mobile and I can see the visualization however I am not able to type any question to do the custom analysis. When I click inside the Q&A box to type a question, I am not able to see the cursor blink and when I try to type, I am not seeing the text that I am type and in turn it is not bringing any results.