1. When importing, the scale option attempts to compensate for the relatively (compared to other applications) small scale of Poser's characters. If it isn't used, the hip translation might place your character three hundred feet above ground level.

2. No. Every frame is a keyframe. It's a limitation of the BVH format. However, after importing a BVH, you can have Poser resample the keyframes.

not to mention how wasteful it is for keys to register on the nonkeyed elements and morphed elements.

Morph channels aren't included in a BVH. Just rotation and (for the hip) translation.