Learn English – “I have a certification” vs “I’m certified”

1 and 2 are both correct, and at least from US usage probably occurs relatively equally. However, the impression is not quite the same.

1. Kristina has passed her exam!

This implies that it was unexpected, or that she had some difficulty passing her exam.

1. Kristina passed her exam!

This is just a simple statement of fact (other than the !).

a. She has got 90%.

You can't use "got" with a helper verb. Should be "has gotten".
But I think "has gotten" is a poor word to use in this context. Something like

Her score was 90%.

is probably better.

They are both grammatical, and in most cases interchangeable.

Discussion is one of those words which can be a mass noun or a count noun. As a mass noun it means the act of discussing in general, as a count noun it means a single event of discussing. So for useful discussions implies that there were several separate times at which you discussed. For useful discussion says that you have discussed, but contains no implication as to whether this took place once or several times. (The third possibility for a useful discussion is explicit that you only discussed once).