I don't think she lacked her faith into Simba. After the tragic news of Mufasa's death and supposedly Simba's "death", she had to live with that semi-true statement for years going up and not having her best friend around her. By the time she found him, she was only confused because him being infront of her eyes and not a hallucination was so sudden at the moment.
When it came to their argument in the jungle, she was lost at why Simba wouldn't make at least one attempt at reclaiming his throne and overthrowing Scar. She obviously didn't know that Simba was currently struggling with guilt that was placed on him for years and she was upset but angry at the same time.
I always viewed Nala as a character who would have faith in anyone as long as they were good in the heart and understanding. She had no idea what to do after they argued and needed help.
As for SP, Nala is supposed to represent a character who was the polar opposite to Simba since she is being herself, having thoughts and making decisions as a queen, opposed to Simba where he was a king who was trying too hard to represent the predecessor, which was his father. He needed to learn how to be only himself. Allowing Kovu to stay in the Pridelands for a while gave us a good conversation between the two opposites and view how she is trying to convince Simba into giving Kovu a chance to prove himself. It's not her lacking faith in Simba. It's her supporting her husband.