That's one thing I never understood. Morrigan says all she has pretty much needed was knowledge and power to open others. And Corypheus doesn't have either? He loses one eluvian and just says to hell with it? Like Morrigan locking Mythal's eluvian even if she doesn't drink.
This'll make my head hurt if I think about it too much.
Okay so lemme explain Eluvians. I know that in the game, you just jump through and end up in Skyhold but that's not really how it works. What happens is, between each eluvian is a path similar to the Crossroads, as it is half the fade and half physical, which you must walk through to reach the next eluvian that leads to wherever you want to go.
Corypheus wanted to enter this "inbetween place" similar to the crossroads by entering through the temple of mythal eluvian. He wanted to then use an immense amount of magic to break down the weak boundaries of the fade in this place to fully enter the fade itself. From there he would have gone to the black city and made himself a god.
Morrigan wanted the well for its potential secrets/knowledge.
Also, I think once an eluvian is unlcocked it can be used freely, as long as it doesn't require a password like the ones in The Masked Empire.
(also, Eluvians are extremely rare and I believe most require a passcodes as shown in The Masked Empire. I believe the one in the temple of mythal was one of the few that only needs a key and conveniently had the key still intact and up for the taking)