d) yes running at near capaicity isn't efficient but 700W won't be even using your 420W example on that board except maybe when you are running stress tests. Running at 35% capacity isn't efficient either. So yes you can have too many Watts
Excellent point, its good to give some room for upgrading in my personal opinion but the 700w gives that, no need at all for a 1000w psu for real life situations.