Remove curse frees the soul from the torment of being burned in an infernal machine but only allows them to become lemures.
Do they turn to soul coin after the devil fulfill his part, or something else?
Again, depends on the contract. The hells are the boardrooms of major corporations. The sample contract in the Avernus module specifies that you become a pit fiend upon a) reaching level 17 and b) dying. So after 17, no more "raise dead" the character is perma deathed. I can't remember what type of devil the guy who sold out Elterel became but it was very low level and his contract gave him a number of years.
Warlocks do not become coins unless the contract says so and/or they are evil. Doesn't make much sense but in 5th you can be good warlock of the fiend and have your soul go onto mount olympus or similar upper plane.
