I agree with you that Wyll doesn't come across as someone with strong will, but I think Larian thinks he's very charismatic. I do find him fun to talk to, witty and amusing. I don't recall what his actual wisdom score is, but I think he comes off as good with words, but low wisdom, so he doesn't think things through, he's foolhardy, and even though he can talk a big game, he doesn't have the conviction to back it up.
I do disagree with you about how Lae'zel is written though. I don't think confidence and charisma go hand in hand the way you imply. Low charisma doesn't mean necessarily being quiet, even in the players handbook it says that a low charisma character can come across as abrasive. I think that's what they're going for with Lae'zel, she's meant to come off as abrassive and unlikeable. Bossy and acting superior in a way that most people don't like. She's confident, but in a way that's meant to feel unearned. In a situation where charisma is actually called for, I totally believe Wyll could sweet talk his way through a situation whereas Lae'zel would just make things worse and escalate it.
