Is the divorce valid? Is what happened between the engaged couple considered adultery? What is the expiation for that, given that sexual intercourse was denied before the judge?

The fiancé is a stranger to his fiancée [before the marriage contract], and what occurred between you is a great sin that necessitates sincere repentance. If "external intercourse" means touching without penetration, then this is not Zina (adultery/fornication) that warrants the prescribed punishment (حد), but rather it is a prelude to Zina. However, if penetration occurred, even with a barrier, then Zina has taken place.

If the act was not repeated after the marriage contract, then you did well by concealing yourself before the judge, because what happened before the contract was a pure transgression, and you must continue to conceal it and use ambiguous language. In this case, the divorce is a minor irrevocable divorce (طلاق بائن بينونة صغرى) if there was no privacy (خلوة) between you after the contract during which intercourse could have occurred.

However, if after the contract there was enjoyment or privacy during which intercourse could have occurred, then it carries the ruling of consummation (الدخول) according to the most preponderant opinion, and you are entitled to all the rights of a consummated wife (full Mahr, 'Iddah (definition), and maintenance). In this case, the majority of scholars hold that the divorce is irrevocable (bain), while the Hanbalis hold that it is revocable (raj'i). You should have informed the judge about what happened regarding privacy or enjoyment. If you did not inform him, then you must observe the 'Iddah, and you are not permitted to become engaged during the 'Iddah, and you must seek forgiveness and repent.