This is very interesting! I actually really love the current development; it feels refreshing to not immediately have the MC fall in love in 5 chapters, as like the other in the genre. The MC herself is interesting. Totally love it, thanks a lot!
And for the current development, regarding the saint girl... I think it's pretty believable. I mean, if someone who resembles the demon lord appear, along with a highly terryfing power level, who wouldn't have this kind of suspicion? What will be weird is if the little miss saint would let go of her suspicions too fast. Then it'll be boring as hell. And besides, we're talking about children here. They're are teens living in their own world; having this mindset is pretty much logical.