The 9/11 attacks can be proven by seeing the wreckage on that day, people's firsthand accounts, etc etc. WWII? I'm sure the veteran's have stories and I'm sure we could still find remnants of struggles from battle locales in those areas.
I understand what you're saying though, why believe anything you see on the internet, or in a book, or anything for that matter when you're not there. History will get scrambled as time goes by. Take the Constitution of the United States. We don't know what the forefathers truly meant when they wrote the Bill of Rights verbatim.
For example, take this Family Guy sketch on the Second Amendment:
Link. Sure, this is just a joke from a commonly criticized TV show, but it does have a point. We all see the Second Amendment as the right to bear firearms, but that might not necessarily be the case as through history it could've been lost in translation to further someone else's agenda, or it could be what we think it is. We have now way of knowing unless someone decides to build a
time machine.
Back on topic, and minus a few videos, the reason I stand out against Wikipedia as a source for reliable information is that anyone, anywhere can edit it. This is a common known fact. One day, someone could decided that World War II was actually a huge fight between Jesus Christ and Robotic Super Dinosaurs from the future (don't laugh, my friend in high school actually did this one day in class.) However, with most other sources on the internet or in encyclopedia's, the writers have more notoriety. We trust they will tell nothing but the truth through this.
So, while the video might have valid points and you corroborating that information through Wikipedia doesn't immediately mean it's fact. Those same people who made the video might've edited Wiki to make it seem that they were true all along, so I apologize if I don't take you at your word for finding a backup for their information on Wikipedia. This isn't to say it's not true, and I still don't wish to watch this video because I hate conspiracy theories, but it does seem like you're building your argument on sinking sand.