MC most likely wouldn't be able to command any of the giantesses like a slave. Perhaps he could skillfully manipulate one of them, yes.
Maryen could do something at MC's request, I agree. Even something that now seems improbable.
For example, if the situation were to repeat itself, where MC was attacked by some other tiny one in Maryen's presence, or whole group of bandits or kidnappers with the intention of selling into slavery, and MC then would be like...
Then Maryen would make them all...flat
View attachment 5472546
Moreover, if, at the time of visiting Homaspernia, Maryen had already been the same as she had become at the time of the last update of her liberation route, the demon that attacked MC would not have gotten off with only an injury to his legs
