Exactly. Not every narcisstic person is successful at manipulating and charming people. Just look at Casey Anthony and her mother Cindy Anthony. Narcissism at it's finest, and they just won't quit thinking that whatever they say is not going to be believed. Part of narcissism is believing, with the utmost stubbornness, that everyone is wrong when the narcissist is the only one right. Why should Terri tell Kaine and Desiree about the appointment? That would take away from the shock and awe factor. No, let them find out about this appointment and praise her for making it - that is what she was counting on, them believing she made the appointment and that she was saving Kyron's life. She didn't count on them totally disbelieving her, and then from there, the lie being proven. She counted on them believing the very words coming out of her mouth.
That's a narcissist for you. They don't expect anyone to go beyond their words and prove them wrong. And even then, narcissists still persist in believing their own words, no matter how much it's proven those words are lies. It's why people like Casey Anthony are where they are. She'll never admit what she did to Caylee. She'll always to want to blame it on someone else because her narcissism impedes her ability to believe ANYTHING beyond her own words. If Terri is anything like her, and I believe she is, she won't ever admit to what she did because she can't believe that her words are lies. Everyone believed them before. Everyone thought she was a good mother before. Why would her words be lies even now? She'd rather destroy everything around her instead of admit that she's wrong and did something wrong. That's just not being a perfect supermom. She can't not be that in her own mind. So the lie has to stand, has to work, has to be truth.
Narcissists can't fathom that their lies don't work. This is why they doggedly persist, even in the face of a mountain of evidence that proves them wrong, to continue with the lies. They keep believing that lies are going to prove everyone else wrong. Casey is still waiting for something that is never going to happen, but that hope is always there for her because she's a narcissist. The same came be said for Terri. If she doesn't say anything, if she never admits that she's wrong and did something wrong to Kyron, then it can never be true in her mind.
She believed the lie about the appointment would work. After all, her lies worked before and look how people thought she was a great mom when in reality, she wasn't. I bet she still can't fathom why that lie about the appointment didn't work like a charm like all her other lies did. For a narcissist, a lie not working just doesn't add up, just doesn't make sense to them.
It may look stupid or crazy to us, but to her, a true narcissist, it made perfect sense because the lie is the truth to them.
ETA: Desiree tolerating her, Kaine not pressing her on issues - she already THOUGHT she had captured her audience. That was her mistake. That is why she thought her lies would easily work.