Try that same shot with the Metz at ISO 400 and f/4 and chances are it would have been exactly what you'd been looking for... camera setting unchanged.
Setting the Metz at f/2.8 will put out less flash than putting it at f/8. At f/2.8, the lens aperature of your camera is open wider and as such will need less flash for an exposure. (generally speaking) ...
Number 2 depends on the scene...not all scenes are the same.
Mark said the Metz fires one stop hot, which I think is totally accurate. Therefore, if you set the Metz at f/4, it's probably firing more like f/5.6
ISO has no impact on shadow directly. Shadows exist from where the light isn't hitting. The only way of removing shadows is to either put some light on the shadow area or change the direction or intensity of the light causing it.
The slower shutter speed will allow more ambient light in. Correct.
Dyslexics of the world UNTIE!
Julio