Learning from failures is fine, but you must agree that there are failures so great, that there's simply no excuse for them. Losing the key is almost the worst case scenario for Calyx' plans; one of the last things he should've wanted to happen. (Remember: He has waited hundreds of years for this key, and that he got it back at all was actually just an insane coincidence)
There is a difference between failure and catastrophe.
For Example: If you found a button that could destroy the earth, would you hand it to the equivalent of an emotionally unstable teenager? Of course you wouldn't, because everyone knows that is not a good idea. There is nothing good to be learned from doing that.
If you introduce a character to a story that is a scientist, driven by intelligence and logic, every time that character does something illogical, it's either a writing mistake, or a plothole that needs filling.
I can accept that Calyx' plan might not be flawless, but if that is the case, the story should shine a light on it instead of making us guess about these supposed flaws.
And that is all I really want in the end, and I really hope 7.3 delivers. There is a good chance that Sphene's crown will tell us more.
I'd argue that there are ways to replace Sphene and manipulate the masses' memories without stirring too much trouble (seeing as the Alexandrians are probably the most docile people we've ever met), but we'd get massively off track that way.