I agree, it sounds like Eliza had "promised that nothing--" would make her give up her resolve to marry Col. B; that she would hold out 'til his father consented to their marriage or until she didn't need his consent anymore.

His story is so jumbled up chronologically -- were he and Eliza eloping to ESCAPE her forced marriage to his brother? or did the father lock her up and bully her into marrying the brother AFTER the failed elopement?

I think Brandon is doing his best not to blame Eliza for having given in, almost to the point of making excuses for her. He loved her, but he was also disappointed at her lack of "fortitude".

I imagine Eliza's position similar to Fanny Price's in Mansfield Park... subject to nagging, bullying, guilt-tripping by guardians who want to her to do something against her will. It's not beating and starving, but it would be hard for a young woman to resist. And Brandon had been "banished," so she had no ally or comforter at home.