I'm not sure if this is true for other phones listed above, but for The Galaxy Note II, I had to make sure S-Voice was disabled (it says Voice Dialer on the first post).
Also, if you have downloaded bluetooth launcher you probably already tried:
"Google Search" and then selected "com.google.android.googlequicksearchbox.VoiceSearchActivity"
and was still launching S-Voice. Once you disable S Voice (note the space) go into
Settings > Application manager and scroll to the right to ALL. Then find S Voice and disable it.
Once you disable it and hit the button on the paired bluetooth device, you will get a "Complete Action Using" dialog. Resist the temptation to choose Google search. I know it's the logical choice, however you will not get Google Now; you will get a Google voice dialer instead. So make sure you choose "Bluetooth Launch" and select ALWAYS. Now when you hit the bluetooth button, you should get Google Now.
Note this will only work for the bluetooth. I had to use a separate app (Home2 Shortcut) for the double tap on the the HOME button.
- Go to the Google Play Store, and download the Home2 Shortcut app
- Open Home2 Shortcut
- Choose "Google Voice Search"
- Choose the speed that you would like to double tap thee home button (I left mine at NORMAL)
- Choose "TouchWiz home" for home application if you have the default launcher. If using something else chose that one; in my case I was using NOVA Launcher.
- Scroll down to the CLOSE button and click it.
- Double tap the HOME button. You should now get a "Complete Action Using" dialog.
- Choose Home2 Shortcut and select ALWAYS. You're done!
FYI: I don't think it matters but I'm rooted with a basic stock ROM. Again, these were the steps for me on a Galaxy Note II (Sprint).