If the marriage is valid, nothing that happens afterwards can make it invalid.
Novus Ordo tribunals are not competent to decide in this matter (and probably any other).
Separation is different from divorce. Separation simply means not living under the same roof, while divorce means that the former spouses are free to marry other people.
Divorce is impossible for a Catholic, since, even if you have a civil divorce, you will be married to your spouse until your death or the death of the other part. So, divorce, per se, is forbidden for Catholics.
Separation, on the other hand, is tolerated for a just reason. In normal times, the local bishop would decide on such matters. If the parts are separated, civil divorce can be tolerated for a just cause. Obviously, none of the parts would be free to marry other people, even if they had a civil divorce.
This is my understanding of the matter. I am not sure that you have understood this concepts clearly.