the button/lever on the shutter at the 4 o'clock position of your picture should hold the shutter blades open for focusing.
no, it's not. There are a lot of lenses made for speeds/crowns that do have a little lever that you push on in that location when you have the shutter cocked to open the shutter.
However, note this shutter is not mounted with the top pointing up, it is at a 90 degree angle to the right, and the shutter release is at the top.
It's a jury rigged setup using the lens/shutter from another type of camera. Since the Speed Graphic has a coupled rangefinder, and was primarily meant for news work, it doesn't really need to be able to open for focusing. That's what the rangefinder is for. That later shutters did include a facility to open the lens was a convenience.
What does this button do? If you cock the shutter at one of the lower speeds, then push it back towards the body of the camera, you can then push the cocking lever of a bit more.
Fire the shutter, your self timer is buzzing away. The button on most shutters like this doesn't do that at the highest speed because then the extra spring in the self timer is being used to make the shutter go faster.
As to B and T -- I'd try pushing the shutter release WITHOUT cocking the shutter first. Most of the similar shutters I have on Zeiss products of this vintage work that way. If they don't work that way, then I am at a loss.