Most folks aren't seeing demons, and especially not enough to do a 2-year secret art project while on drugs. It is, honestly, more common for a businessperson to have a secret drug habits.
I'd also like to point out that folks have seen demons for quite some time in history without having it fueled by drugs: Religion can fuel this, especially when combined with little understanding of the observable world around you.
And lastly, just because you don't see anything demonic nor disturbing doesn't actually take away from it being such to others. I put random eyes in artwork sometimes: I like it, but it really makes some folks' skin crawl. They definitely have different opinions than I do.
If the conservator was under the influence of drugs or alcohol for that long, I would imagine signs would have showed up in his other jobs, in his personal documents, or in other aspects of his life (the vicar didn't note that much substance consumption?)
Regardless of the cause, I'm quite interested in what the conservator's reaction to it would have been if/when he realized he did not actually conserve the original art. I've watched a few art conversations (Baumgartner Restoration on YouTube is great!) and there's a huge pride in accurately reproducing/restoring the original.