On the subject of the potential loss and nearly impossible-to-recover nature of Mt. Rushmore, there is another possible solution.
Give Americanists the same event chain Atomicists have at certain chosen sites - the ability to spread spontaneously, whether or not the feudal lord above them supports it or not.
The obvious areas for this ability, like Mt. Rushmore, Washington D.C., and so on are already under Americanist control at game start, rendering it mostly irrelevant. However, if Americanism is struggling to survive as a religion, this could allow it some areas to regrow and bounce back with new leaders.
This may also give the Americanists (one of the most fleshed out and iconic religions of the mod) more game presence, but in a subtle, enduring way (instead of throwing large Americanist empires on the map or similar). It would help the possibility of getting presidents from different regions and cultures as well.