So I managed to complete that quest and save the president. But not in the way that the Fallout Wikia guides have said.

I climbed up to the vertibird after Kimball has started his speech. I tried examining the vertibird but didn't have enough skill to examine it. The people near the vertibird started attacking me so I had no choice but to kill them. Then, the vertibird flew upwards and out of sight and I got a quest update saying that I protected Kimball successfully.

Kimball was no longer on the stage and the audience were all in their underwear.

Apparently, I was supposed to stop three attempts but the quest ended far too early. Just what happened?

You went up to the roof without having asked for proper access, so you trespassed and were shot at. You were basically attempting to assassinate the President so he fled, he escaped. Well done?

But it says that if I trespass, I automatically fail the quest?