1. Yep. The flash duration will be a lot shorter than 1/125.

2. Maybe, or close the aperture from whatever the flash recommends by the factor of stops your faster film is than 400. In this case 2 (400->800->1600) So if the flash said f5.6, use f11.

3. It is probably use to spread the light in a wider arc, eg for when using a 28mm lens (my flashgun has something similar)

4. Bounce the flash off the roof. I think a 285 is powerful enough to do this. Aim the head almost vertical if you're getting fairly close to the subject.

5. Experiment (options 2-4 really require an experiment or two before using)