No, a perception alteration needs to be maintained consciously or set in a self feeding loop.
Mass Illusion makes something that isn't. Lasts till it runs out of energy.
Could be a projection.
Main difference is whether it exists as a magical shell or an internal change. Mechanics difference, really.
But yeah your example of a room being full of spiders would be...
Hellishly difficult with actual Projection.
Since you'd have to make an individual shell for each spider and spiders have a lot of little details that need to be put in them already.
Though you could just make little balls with stick-legs.
...
I now have an image of a Yakko terrorizing a town with crudely drawn stick figures that run around and poke people all day.
Though the amount of magic that would be needed to actually animate something like that would still be massive, given you have to make so many finely formed objects.
But yeah think about it more like this.
Someone who can use Perception alteration can make a stick look like a sword, but if you hit someone with it, it's still a stick.
Someone who can create Projections can make a stick look like a sword and, given enough skill and power, make it
cut like a sword, too.
the spider thing is actually way easier to do with Perception Alteration, since you just lay a directive that 'this room is full of spiders' and either tie it to a person, in which case that person sees the room full of spiders until you lose control of the illusion, or you tie it to the room and invest it with energy, meaning that anyone that sees it is hit with the room being full of spiders until it runs out of energy to produce the effect with.
Basically, from what you've said, your illusions work by overriding perception, right?
Does that change with Mass Illusion, or is it still just overriding perception, just on a larger scale?