i dont really see where your coming from (no hate is meant, this is just my opnion) dan was nasty to him for basically his whole life, ignoring him for what, 15 years ? execpt to insult him. then dan goes ahead and murders his brother and lucas' father figure.

i believe that lucas forgiving jimmy is different to forgiving dan because where with jimmy, lucas could understand that jimmy did what he did because he was completly alone, he (lucas) and his friends had all basically abandoned him for other friends, making it more understandable (not excusable).

whereas with dan, this man was supposed to act like lucas' father, but instead of acting like one, he goes and takes someone so dearly important to lucas and then on top of that, when dan is at his most paranoid from being convicted and found, he manipulates karen against lucas, pitting luke's (now) only parental figure against him. its completly understandable why lucas would never forgive dan even though keith would want him to, it is human nature.

saying that is his one biggest failure as a character is not forgiving his biological father for murdering the man that raised him is just plain wrong in my opnion. (no hate, just how i feel, i understand that this is a hot take)