IMO the drop rates of tokens should be same for all the difficulties and should reflect the ratio of each troop rarity. For example if there are 850 troops in the game, and 50 of them are mythic ones, then the chance of getting the Nysha token should be 50 / 850. Similar for the other ones.
I do not understand, why lower difficulties have no chance to get Nysha. Even low end players can have mythic troops, so why not to give them a chance to upgrade them.
For those who will argue that if the token distribution is the same for all difficulties, people will just farm low ones, I would like to remind, that give us much fever mythstones and because of that we need to play more full explore runs to get to the mythic.
As for number of tokens rewarded I would go this way. Level 1 25% chance, Level 2 50% chance, level 4 100 % chance to get 1 token. Levels 5 - 8: 1 guarantied token plus 25 - 100 % for the second one. Level 9 - 12: the same with 2 guarantied tokens.