Have you had any luck, by chance? I'm having the exact same issue and can't seem to find any answers to it. Since the update my Galaxy S4 has done its best to forget S Voice exists. Even when the screen is unlocked it now launches Google Now from my bluetooth instead of S Voice, and when it is locked it uses the almost useless google dialer. It's been extremely frustrating looking for an answer because every single search result for this problem, even if it is the exact same question, is people giving instructions on how to replace S Voice with Google Now. I agree S Voice has some hair tearing problems, but I very often use my bluetooth headset for handsfree text messaging, and Google Dialer has no options for this and Google Now is terrible for sending and receiving messages when I can't look at and physically interact with the phone!
I just can't seem to find a way to make S Voice the phone's default from either locked or unlocked. I tried using an app called Bluetooth Launch(suggested for the purpose of making Google Now the default of course), but that way didn't work either. Has anyone found a solution for this yet?