Let my try to pose a simple question. I’m interested in how the “metagaming” crowd answers this.
Because they failed their saving throw, our PCs fully believe the corpse to be real, as in not an illusion. It looks and feels real to all our senses. The question is, can our PCs believe the corpse to be fake even though it looks and feels real in every way to them.
Because this DM runs illusions in a special way, the corpse fooled all our senses at first without even allowing a saving throw. When we said we interacted with the corpse the DM implied it was perfectly real. We felt the resistance as the sword cut through his neck, we felt the weight of the head as we picked it up and moved it from the body. “As players” all of our physical senses pointed to the corpse being absolutely real and corporeal. And still, we decided it was a fake.