Another thing to keep into consideration is that the Susan in Grave Peril and before is not exactly the same Susan as the one we see in Changes. They both learned a lot and they both got older. Their perceptions and priorities changed and they had true love, technically broken by Harry with Lucio but I do not think that made that much of a difference emotionally when they met again. That love must have colored everything they felt about each other.
So to understand Susan in Changes we must not look too much at the Susan before Grave Peril, that was very long ago for her, but look at the Susan in Changes. What that Susan drove was the same as what Harry drove, an all consuming wish to protect their daughter. She wanted to defend her actions, most of us do, but she mostly wanted that past out of the way as soon as possible to start what was really important.