Personally I'd rather have a long press button activate it. Saying "OK Google"or any derivative is a waste of words me. Apple got it right with holding the home button to activate Siri. I'm really sure why this doesn't exist on Android as it seems to lower the friction quite a bit.
It does. Long press on the home button, or press and swipe up.
I might be doing it wrong but it might also be that I'm on a Nexus 4. When I do that it brings me to Google search but the voice prompt still needs to be activated via tapping the mic button or using the voice command. Is there a way to change that to just immediately activate it?
And what about from when the screen is off? I have a software home button so can't use it until the screen is turned on unfortunately.
Screen is off your only option is to talk to it. What version of OS are you running? Might not be backwards compat.
Double click home. That defaults to voice. Long clicking also takes me to search.