The Ministry of Magic's formal trial of Umbridge came as no surprise to Albert. Harry Potter deserved most of the credit. At Albert's prompting, the young Savior had seized the best possible moment to accept an interview with reporters and dealt the Ministry of Magic a brutal blow, forcing its politicians and bureaucrats to abandon Umbridge.
Yes, abandon her.
Although Umbridge had done a great deal of terrible things, she had only been an executor. She had acted on Fudge's instructions and, fundamentally, worked for the Ministry of Magic. In this matter, she should not have borne all the responsibility.
But once such matters were exposed, they would inflict irreparable damage on the Ministry's credibility. Besides, in the eyes of most politicians and bureaucrats, abusing one's authority was not really a major crime.
Umbridge was an old hand at the Ministry, powerful and high-ranking in her own right. She was capable, and she knew how to ingratiate herself with people. In the novels, after Scrimgeour took power, he naturally would not have dealt with Umbridge in order to better control and stabilize the Ministry. Besides, Scrimgeour had more urgent matters than Umbridge to deal with.
After Albert's meddling, Harry Potter had flipped the table outright. As the Savior with the greatest influence at present, he was showing signs of guiding public opinion against the Ministry of Magic. For political reasons, Umbridge was naturally discarded. Otherwise, who knew what Harry Potter might tell the reporters next, or what else might happen.
As for the Ministry deliberately scheduling the trial for the weekend and inviting Harry Potter as a witness, that was merely formalism. While convicting Umbridge, they could also win Harry Potter over, demonstrate the Ministry's fairness to the public, and strengthen its credibility.
But Albert found all of that laughable.