> Do we assume he twisted each blade 90 degrees before installing in the frame?

Well, since we see it being shredded, I'd say "yes", if you remove your qualification word "before", or clarify meaning by inserting the proper word after "installing". I can't tell if you mean "before installing them" meaning "the blades" or "before installing the painting".

They might get installed in a "safe" way so that they won't shred if the motor somehow turns on without the blades first being armed. Or maybe they are intentionally installed that way and a shaped roller pushes them against the blades, even if they are laid flat.

In short: who cares -- it shredded. I don't care if the full mechanism or method is shown. It clearly worked, at least partially.

I agree. But as someone very interested in how physical things work, I can't help but think the video we were shown of the creation is not of the actual shredding device ultimately used in the painting. Blades like that don't bend very easily. But if that is the case, why? Why make an additional fake video of how the shredding mechanism was created in addition to what was really used?

The blades needn't have been twisted as their tips were sharp pin points. So, pulling paper/fabric across just tips in would have resulted in cuts.