For the first, when she goes in the spa and decide to get naked, it's just after her session at the shrink, that told her she needed to liberate herself (in so many words), so here she is at the spa (which is a little bit more than a spa), in a safe place where nearly everyone is naked, making a step towards doing what her shrink told her is easier here, she gets to try to be a bit naughty, but in a safe environment (it would have been something else to do it at the pool or the beach).
For the mummy, it's something else, first it's a "weird event", you have to see it as an mini adventure in Myriam's big adventure, they are like little spin-off inside the game, so don't take them too seriously, they really are just here for the fun; second, the mummy is using "magic" to control her, and contrary as you think Willpower did had an influence on this scene, with a high Willpower, Myriam could resist in part and see through the illusion, giving the scene a few more renders and a few different lines, it also change a bit the ending, instead of freeing herself by accident, she do it because of her strong will.
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So no matter what Miriam's stats are, what Dr. Silver tells her, She's going to do it right now.
