Why does the narrator decide to introduce us to montag at this time of his life , when he encounters clarisse and confronts mildred's overdose?

The narrator decides to introduce Montag at this point because this was when he started thinking about his job as a firemen. especially when he met Clarisse she asked him if he was happy. At first he brushed the question off but later on when he was burning books he realized that he wasn't happy after all. Overall, This was the part when Montag starts realizing that he needs to change for the better.

is that lennie, a man with a mental disability, is treated poorly due to his mental disability

you need need more details so we can understand what your asking us.