I wouldn't say it's only a "theory" that remnants of a 16th/17th century (not 14th) mural depicting biblical figures were present before Gotaas turned up. That is clearly documented. I mean, you could make a very far-fetched hypothesis that under this work was another, much older work, unlike anything else ever known to be produced at the time, which Gotaas somehow discovered and restored without telling anyone else and without stopping to ask whether they would prefer this to be restored instead of the biblical scene they had originally asked him to take a look at. But there is absolutely zero evidence that any of the figures depicted by Gotaas were present before he began his "restoration".
I'm aware it's far fetched and I don't even really believe it, and there is no evidence like you say, but it's also kind of hard since there doesn't seem to have been a huge amount of analysis done before his work. To me this is one of those things we'll basically never know the answer to; the crucial information(state of the mural before the restoration) is lost to history. It reminds me of the Voynich manuscript in that sense.
I was just captivated by the idea of some 14th century priest losing his mind and doing this. But yeah, not exactly likely.
