Just use veritaserum on Harry and all is resolved
Hey Harry, did you actually need to use a spell in self defense against Dementors? Oh wait, let's drug you with veritaserum and find out.
Oh did you also happen to actually witness Voldemort coming back? Cool, no reason not to believe you then. Let's all unite against him.
What's that? There are people still unsure that what Harry is saying is true? Ok, just use the Pensieve and we can all literally see what he saw.
Oh, maybe he tricked both ways? Ok, let's just use Imperius Curse on him and force him to tell the truth.
I mean jesus, so many ways to solve that stupid trial in the beginning. I'm sure Harry would have been ok with either of those methods to prevent being expelled from school. It's not like those methods hurt or anything.