I haven't played a game in which a rival nation was doing the spell of mastery or the forge quest so I don't know if a warning of another nation beginning these is provided. But, if not every nation should get a warning when one of their opponents  begin or get close. It would make sense if when you got the warning depended on your diplomacy tech or ratings with other nations who might warn you if you're warm and friendly or as a Channeler you might detect a disturbance in mana from the beginning of the spell of mastery if you have enough spell knowledge or your spies and traders might give you a rumor that they heard.

As a consequence of getting warnings there should be ways to interrupt, add difficulty to, or just thwart the enemies plans to win the game. Maybe a sufficiently advanced channeler can cast a mana static spell which slows the spell of mastery to half speed in casting it or a dispel spell of mastery spell that allows you to completely reset it or reverse it five turns to completion.

You might have spies steal or forge important documents and let them fall into enemy plans causing them to waste time on wild goose chases (this would work well with my idea of using question marks on the map for fuzzy knowledge). Maybe you can kidnap scholars or your espionage agents can set fire to a critical research lab for example.

Basically you should have the chance to make your opponent wince or hope that he can slip a victory past anyone without them noticing. Uncertainty adds volumes to a game and daring moves both to push for victory or to counter it make the game more interesting. I could imagine a sovereign abandoning defense of cities in an all out push to finish the forge and to have enough troops to take out the hermit while his opponent is going "what an idiot for not defending his territory!"