Is it possible for the app to recongnize that you are moving in a vehicle. Possibly using some background speedometer and turn on mobile mode if you have that checked in the settings.
Then when you get a message a notification, a box pops up and ask "if you want the message read aloud.
If yes is selected it will read the message then afterward ask you if you would like to reply (just yes or no buttons)
If yes is selected then it will allow you to speak your reply and tap the screen when finished.
Then it asks if you want to send it. (you can tap yes or no button)
The buttons would be big enough to make it easy to choose while driving

I would be up for beta testing if you think this is a good idea

Thanks