] Could Edward's prolonged engagement to a completely unsuitable girl be a form of secret rebellion by him against his mother?

In a twisted way, I suppose it is rebellion against his family to behave honorably in the face of disappointment, since his family never behaves with anything like honor throughout the book.

Edward's duty to Lucy Steele is one of the trickier things for modern readers to grasp. But I don't think it was motivated by rebellion. It was motivated by the fact that he was an honorable man and it was the right thing to do. Had he done otherwise, he never would not have kept Elinor's heart. She would not accept a man who wormed his way out of a solemn promise, even if it was done so that he could marry her. If Elinor is to love him, he is completely trapped, only to be released by Lucy's cold duplicitousness.