I never saw it as fixation. I feel like he's concerned with her because she reminds him of what happened to his old love and he fears she will suffer a similarly bad fate. But, also, she is with Willoughby, and Brandon knows at some point that Willoughby did get his charge pregnant and didn't marry her. All of that would be very concerning.

Yup, yup, and yup. I think you were paying attention to the plot, and less to the age gap, which is what seems to squick some people.