Please never put any secret “hard modes” in the game again. Properly separate them from the normal game mode.
A 100x better way to implement it would be to make it visible in the matchmaking only for the people that completed the normal mode.
The way it is now, people that want to do the hard mode are put together with people that dont and vice versa which is just a horrible experience.
The hardmode could be in a separate mm queue, but should not just pop up once you clear normal mode.
The bells being rang is completely fine as an unlock requirement.
So the hm could pop up after you do both:
finish the mission on damnation normal mode
do the bell puzzle (which would no longer turn the normal mode instance into hard mode).
Alternatively, the HM could be unlocked by doing the bell puzzle, but you only get access to the HM on any diff equal to, or lower than the highest diff you have cleared on normal mode.
One of the two unlockable bells at mission start could then be removed, since there would only be the need for one boss teleporter, with the normal and hm queues being separated.
The teleportation bell could then be unlocked by clearing the mission (or reaching the boss room) on hm.