You seem very focussed on the idea, that a character is only defined by explicit dialogue telling us how a character feels. Yet, a character is also defined by his or her belief system.
Personal inference and interpretation is a big part of film experience (for me too). However, unlike people around here, I do not use it in arguments because then it becomes as pointless as arguing which colour is prettier.
Belief system is of course an important part of the character. By far the best manifestation of it are the actions that the character does. The dialogue is less so since the character could “talk the talk but not walk the walk” but still at least it is unambiguous. The least reliable are interpretation from hints and so on, which are completely subjective and dependant on the viewer.
By the end of TESB it is very clear Luke’s belief system has been turned up side down. As such his character cannot be the same.
Well you might interpret that his belief system was turned upside down by the end of ESB and that he wanted to go back to Yoda instead of going for another friend-saving adventure. However, his action at the end of the film suggest otherwise.
Well obviously Luke’s friends still mean a great deal to him. Character development does not have to be black and white. He’s been through a harrowing experience, but one of his best friends is in mortal danger. The fact that the film ends with preperations for a rescue attempt should not be taken as evidence of Luke’s lack of character development.