I read a quote by Rav m Lubovitch (A"S) that regardless of how right or wrong one is, a husband and wife should make every attempt to repair and preserve their marriage and Shalom Bayit. At what point in this effort, does divorce become the proper course of action?

It's a tough one. It is an individual decision, there's no one answer to such a question. When the marriage really can't be saved divorce takes its place but only someone that knows the specific case in depth can determine whether there is or isn't any chance.