Harmony - Troop Symmetry

Mages have a library that reveals one spell per level – why do you doubt that adding these two earth-based spells for the Elves is feasible through building construction?

Because this library is implemented in the game's code, and its functionality is somewhat different (a random spell instead of a chaos spell, as you suggested). That is, implementing something like this for the Elves would require scripts, and I don't know of a script function that adds spells to the guild. And I'm afraid that such a function doesn't exist at all.
Maybe I'm overcomplicating things. But at first glance, there seem to be several problems that cannot be avoided. However... It would be possible to create a building that would give spells not to the guild, but directly to the spellbook of heroes entering the city. But that would be a bit awkward.