Well the flattest .308 is going to be whatever pushes the heaviest boat tail fastest. Not that flatness matters too much at 1000yrds.
You mainly want the highest BC possible to minimize drift and energy loss
Negative. The flattest trajectory will be a lighter bullet pushed at a faster muzzle velocity.
A 155 gr Lapua Scenar leaving a barrel at 2,950 fps will have a flatter trajectory than a 175 gr SMK going 2,650 fps.
The heavier bullet will usually buck the wind better, however.