Difference between 妖魔 and 妖怪
As I write this, someone already left feedbacks for your second question. 

As for the difference between 妖魔 and 妖怪:

妖怪: like a monster, its appearance looks quite different from other normal creatures (at least from what's in our minds).  

妖魔: like a demon, having witch kind of looking, who can use the magic power in combat. 

In short, 妖魔 has more magic power than 妖怪. 妖怪 is more powerful physically than its magic power. 

This is my understanding and impression and might not be that accurate. Just as a reference.