My phone is running on Jellybean 4.1.2, which means the solution to your problem may be a different solution for mine.
But, what I can suggest for you is to either disable Google Hangouts, as well as the SMS function therein, hopefully isolating Google Now's default SMS setting to Messaging, i.e. the stock Android messaging app.
Other than that, you may find doing a Factory Data Reset may do the trick. Just keep in mind, you may want to go into your settings and manually back up all of your personal data, like contacts, musics, photos, etc. as well as your Google Account just to make sure.
I've done this before to fix my Android phone a little while ago. If done right, you should still have all of your personal data automatically synced and ready to go by the time the phone resets itself. I'm going to do the same tonight, just to see if it helps.
If it doesn't, though, and you MUST HAVE a way to text message people using your voice, execute web searches, or anything else like that, then "Dragon Mobile Assistant" by Nuance is a more than viable alternative. I have it currently set to a double press of the home button thanks to a third-party app called "Home2 Shortcut". It works very well and is easy to set up. I really hope this helps.