I get what you're saying about the sub path, but I see it a bit differently. While you are right that there's no reason for Mia to trust Valerie over Red Mia, the sub path (so far) is about wanting to be free of Red Mia. So even though Valerie admitted to being the reason why she awakened, Mia now sees submitting to her as the only remaining option to achieve that goal.
It's not that Mia trusts Valerie to not rape her, she knows full well that Valerie's "treatment" involves sex but she's willing to endure that in order to protect her friends and family from Red Mia. That seems to be the point of the sub path, to give up control to Valerie in order to stop Red Mia from taking over completely and possibly harming the people Mia cares about.
Is it possible that Valerie might end up doing the exact same thing to Mia's friends and family that she's trying to prevent Red Mia from doing? Yes, of course.. It's easy for us to see that that's the case. But just as you think Red Mia is the lesser of two evils, Mia herself probably thinks Valerie is the lesser of two evils, due to the situation she's in. Red Mia might not rape her, but she will go after Mia's friends and family, so she basically sacrifices herself to not let that happen.
Because of how we're being shown that Mia feels like she's out of options, the direction of the sub path so far does make sense to me, both from a narrative perspective but also from a gameplay perspective. Because choosing the dom path means giving in to Red Mia (who really is just a highly dominant version of herself), and by doing so basically keeping control. The sub path would then of course have to be about Mia giving up control to someone else, otherwise there would be no reason for there to be a sub path in the first place.
You could say that the sub path could just be about Mia submitting to other people instead of Valerie, but then there would still be the problem of Red Mia slowly taking over. Because of that and everything we've seen so far, it makes more sense for Mia to submit to Valerie since she thinks it might be the only way of "locking away" Red Mia. Whereas taking on a more submissive role with other people wouldn't really serve any narrative purpose at the moment.
That's my take on it at least.
As far as the dom path goes... Well I agree with you, but you already knew that since you've read my message so there's not much else to say about that.