How is missile evasion intended to work? Should you always need to use countermeasures, or is evasion also intended to be an option?

Yes, of course. It would be pretty frustrating if, just because you run out of countermeasures, you always have a guaranteed hit from a missile. So evasion will definitely be an option. It depends how fast you can accelerate away from it and finding the right point when to accelerate away. But yeah, it's definitely an option.