I did not read clearly the first time through.
If in fact the camera is firing when the shutter release is on a half press, then the button/electronics is the problem IMHO.
(But, it's not mechanical it's got to be electrical if the portrait is doing the same.)
However, maybe we are still looking at having turned off the metering associated with the shutter release. If it does nothing at the half press, how do you know when the half press has occured, and what stops one from getting all the way to shutter release if the half press behavior is turned off?
Just a theory