The 'fake old' footage was pretty obvious
It didn't take long to figure out that most of the "Super 8" type home movie footage was actually new footage, using actors to play family members.
The young Michael's hair and mustache were last seen in the Beastie Boys' video for Sabatage.
Some folks on the board feel cheated by this and others are trying to find all kinds of deep artistic motives for the use of this technique. Personally, I think the film-maker just wanted to avoid using non-stop shots of talking heads!
She was solving a simple problem of not having much visual material to work with.
On the other hand, if the shots at the end of the film showing the creation of this footage were supposed to be a twist ending or a big reveal, well there was no surprise at all.