In Silas Marner, what had been decided about Dunstan?
I am not quite sure what your question refers to. We hear no more about Dunstan from when he steals the money of Silas Marner until we find out what had actually happened to him in Chapter 18. People had supposed that he had joined the army or some such scheme, and thought no more of him. However, it is a shaken Godfrey Cass that returns to his wife with the news that his brother Dunstan, after stealing Silas' gold, had fallen into the stone-pit. He had all of the gold with him, as well as Godfrey's hunting-whip. Everything was gathered up and taken to the Rainbow Inn, so it is clear that Dunstan's role in the disappearance of Silas' gold cannot be hidden. However, what is interesting is the impact this has on Godfrey. he says:
"Everything comes to light, Nancy, sooner or later. When God Almighty wills it, our secrets are found out."
This secret that is revealed thus prompts Godfrey to reveal his own secret: the real parentage of Eppie and his former marriage.