Key collision in scrypt and hkdf
No, you don't have to worry about collisions.

As long as no pair of users have the same LowEntropy input, they will receive different MasterKeys.  If the MasterKey is different, then the AuthKey will be different.  Even if you use the same MasterKey to generate multiple AuthKeys, you don't need to worry about collisions: as long as the keynumber values are different, the derived AuthKeys will be different, too.

You don't need to use scrypt if you are using a 128-bit crypto key (assuming the crypto key is kept secret and not revealed to others).

I don't understand what you are asking when you talk about using multiple times the same ECC key.  I think you should post about that separately, as it sounds like a separate question.  (On this site, we want you to post one question per question; if you have multiple different questions, please post them separately.
  Don't forget to read the FAQ.)