Closing your eyes isn't an act of "pure" will, it's always attended by other sensations. If you could will away the yellow disc without seeing black there instead and feeling your eyelids compress, then you could will it away by pure force of will.

A demon would not be required to be consistent, but it appears we can detect patterns in sensedata.

It's possible that reality is just an infinite flux of random events, with rare islands where coincidence gives rise to the illusion of consistency, and we'd still be able to detect patterns if we were in fact dwelling on such an island.