Learn English – “Happen to know” meaning

1. Never is short for not ever. Ever could be used in both positive and negative sentences as in "Don't you ever tell me a lie again".

2. It means I am sure (confident) that it won't happen. However, I would say "It could never happen" rather than "It couldn't ever happen" as using never is more idiomatic than using not ever separately. The reason not and ever are separated in "Don't you ever tell me a lie again" is ever is used for emphasis as No. 4 definition indicates in the link.

If you contrast "I have never been to the U.S." with "I have not ever been to the U.S.", it would be easier to understand the difference. The latter is rarely (almost never) used.

Could in the sentence doesn't have any past (tense) aspect. It is used as a modal verb to indicate possibility.

In this sentence "just fine" means the same as "ok." It is slightly less positive than "well." It can also indicate that the person speaking is feeling defensive in response to the question.