OP: I've wondered about the same things.

Early bike discs were similar to performance autos; lots of cooling holes, and if slots, like they do on cars, the slots are angled to be like a spiral, not perpindicular to the pad surface. On recent discs I have seen, the cooling holes (polygons) seem excessively large, with edges close to radial, so abrupt transition with the pads. I would think that wears pads fast, as it's not only friction, but possibly some shearing.

Regarding outer periphery shape not engaging with pads: That could be just styling (which happens more than you think on what should be engineered components), or additional non-swept heat sink, or there to prevent resonance; Treaded car tires, have the tread blocks and spaces angled different to one another by a tiny amount, to not have them all generating the same noise and vibration frequency when rolling at high speeds, to reduce perceived noise.