On the topic of fake reports, I've already ran into a report with a random video because the reporter didn't have a youtube channel. I wouldn't doubt if others have encountered this as well and I'm sure there will be more in the future.
Now, on the topic of length of videos, I prefer shorter videos as I can more efficiently go through reports. In my experience, editing videos doesn't take all that long. Also, I feel that it's a bit weird when someone would have enough effort to record but not edit the video to be within the time limit. In the past, longer length videos were allowed and many players wouldn't leave the important information that was needed in the post. Then, when asked, only a fraction would respond again.
My non-staff opinion is that limiting the videos to 5 mins is better than allowing any length videos. It keeps things relatively short and helps reports get dealt with quicker.