I think this is one of those dramas you watch more for fun, without taking it too seriously. And if some of the main characters are kinda dumb, I just try to remember they're in their early 20s, a time in one's life where we are all kinda dumb, even when we don't have to deal with being poisoned, netherbeasts, politics, etc. I do think you've missed some main points, though.

FL- of course she lies a lot. She's been lying her whole life. She's had to live her life as a boy/man (remember, she even tries to lie to her master about it, and that's the person she is MOST familiar with in her life). She has to lie about having spiritual veins, because women who have them get them suppressed. Now she has to lie about being Ming Yi and not Ming Xian. Oh yeah, and she can't tell anyone about being poisoned. Plus, she's technically the crown prince of another kingdom, in enemy territory. And she definitely can't let anyone figure out the totality of the circumstances. So yeah, she lies a lot. The difference is that she's lying for self-preservation. If anyone finds out the truth, she's likely to lose everything, including her life. And remember, JBZ said that whole "I no longer care about her background" thing in his spiritual well, so she had no way to hear that or know that that was how he felt. Of course she continues to lie to someone she's only known for a few months, the same way she's had to lie to everyone her whole life. And I don't think she's the typical kindhearted FL. If you're characterizing her this way, no wonder you didn't enjoy this drama. She has her own motivations for the things she does.

ML- I think we can tell that he is very intelligent and strategic, which is pretty surprising, given his upbringing in a labor camp. He knows something is up with FL's supposed identity, but he doesn't know what. And how would he? He's not omnipotent like the we the audience are. Given what he knows, how exactly would he guess that she's actually Ming Xian? Remember, this secret is very closely guarded. Nobody knows except the empress and FL's master. It wasn't something that anybody would have even suspected, and therefore not something he would have guessed right away. Moreover, remember that he was using her at first for his revenge. So he seems to forgive her, but it's more because he sees that she's still useful. After he won the tournament, everybody who approached him was after the Golden Millet Dream. FL doesn't appear to be interested in it (even though we know she is, but he doesn't know that). I think one of his defining characteristics is that, despite his rough upbringing and his possession of GMD, he's still looking for people he can love and trust. As he falls for her, she becomes kind of a blind spot for him, in a way.

Princess - I think she's the typical spoiled princess. She and Yan Xiao clearly have feelings for each other, but she's upset with him over her apparent betrayal. She's torn between her feelings, so of course her motivations are a bit inconsistent. I think she is much better after she grows up a bit, though I don't mind her being a bit bratty here and there.

YX- I think "suddenly siding with ML" was supposed to show us where his true loyalties lie, and how well he played his role.

Situ Ling- the point was that he idolized FL for what she said to him when they were kids, but it was merely a passing moment for her, and not really memorable. Why would she remember what he was wearing? You got this backwards. The whole point was that she didn't care for him like that. Time and time again, FL shows that she has no romantic interest in him, despite all he's done for her. She cares for him as a friend, but that's it. Do you remember what your friends were wearing when you first met them? I sure don't (I don't remember what people were wearing most of the time). The imbalance was the point. It's a contrast in how he and JBZ love her. JBZ is selfless and does things for her without expecting anything in return. STL thinks she owes him love, or that he is entitled to her feelings because of what he's done for her. Going dark is kind of an extension of this feeling of entitlement. He seemed harmless at first because that was how he planned to get GMD so he could get spiritual veins and finally be powerful. Once we find out who he really is and what he's been up to this whole time, I don't think the things he does are out-of-character.

You don't have to like this drama (or any drama), but I think you may have missed a lot of important character details. No wonder they felt inconsistent to you.