One thing to try rather than prying up the contact switch is putting something on the bottom of the trackball. If you pull the trackball and the contact switch is springy if you press it with something pointy, then the contact switch isn't the problem.
I added a small piece of plastic to the underside of the trackball, held with a small piece of scotch tape. Pictures attached.
There's a slight circular bump in the metal on the bottom of the trackball housing. The edge of it is just barely visible underneath the plastic in the blurry picture. The piece of plastic just adds to the bump without having to physically bend the metal.
Don't use anything too thick or it will put constant pressure on the switch. I used one of those fake credit cards that comes in credit card offers. Pretty clumsy cutting, but although it's too big, it worked.