there are so many questions smashed together here. so lets start from the beginning: creating a private key.
to create a new private key you have to choose a random number between 1 and N (which is a little smaller than 2^256 but lets skip that for now) you can't choose any arbitrary size bytes like 99 bytes, it needs to be close to the final size which is 32 bits otherwise you'll have to reduce it and it can create bias (which 99 bytes would).