Why did Harry consent to Umbridge's detention?

[unbelievable]

It makes zero sense to me why Harry consented to the detention given by Umbridge. I would have taken the issue up with the Ministry of Magic. As far as I understand, the wizarding judicial system, just like the muggle one, functions on the premise of innocent until proven guilty, not the other way around.

There was no way for the Ministry to prove that Harry was lying about the graveyard incident, but Harry did have ways of proving that he was telling the truth: truth serum and mind reading. Truth serum can make even the most powerful wizards tell the truth (according to Snape, even Voldemort himself was not immune to truth serum), and mind reading was also a very credible way of finding out if a wizard tells the truth. Harry could have publicly demanded that he be interrogated under the effect of truth serum and that the minister himself read his mind. 

The usual counterargument I see online to this idea is that memories can be altered so that a person can believe things that didn't happen. Fair enough, but while that may have proven that Harry's memories might have been altered (by another wizard, mind you, because no wizard can alter his own memories), it would have stil proven that Harry was not lying about the fact that he truly spoke the truth about what he believed in regards to the graveyard incident. This would have been clear proof that he was not insane or seeking attention, therefore making Umbridge's detention unjust and invalid. No one can be punished for stating something that he truly believes happened, even if what he truly believes happened is the result of having his memories altered.

So, Harry accepting the detention given by Umbridge was a plot hole.