For applying a bit more force, this simple tool design (and photos) is courtesy of Wil Ferch. I've had broken inners and broken outers (happily never at the same time) on my race motors, and something like this pinpoints them quickly, as you can easily notice the difference in force needed for any depression.
My experience with one of the two valve springs breaking is that it acts kind of like a rev limiter - the valves start floating (and the engine stops reving higher) at low enough revs that I've not damaged anything. But in each instance it wasn't just one broken spring, it was four or more.
John Walker must have fingers of steel.