... But Molly and Sarissa are slaves ...
I think it's not exactly "slavery," for example their movement is largely unrestrained. They can travel the world wherever they want, whenever they want.
But, I take your point -- there is no "I quit" clause in the Mantle.
It's more like being part of a criminal gang (that kills you rather than letting you quit); which makes Miss Carpenter a gangster Moll...

... They can't even easily escape it in death ...
You've overlooked Hallowe'en (and other conjunctions). The Ladies -- if they seriously wanted out of their jobs, badly enough that they'd rather
die than continue en-Mantled -- have a chance
at least once a year; plus an unknown-to-us number of other occasions (but I presume less-common, or Samhain wouldn't be Such A Big Deal for the immortal community).