1) Should I say anything to speed up the S? From what I am hearing, there's a good chance she won't see her need to change, or come anywhere close to wanting to reconcile, until S happens.
No. Read Dobson's "Love Must be Tough", I think I suggested it earlier. It will address this. Basically in Dobson's terms you need to throw open the cage door, let her know you support her in whatever decision SHE makes. But it is HER decision to make. Your attitude should be that you want her to stay and work on the M, but you respect her wishes and if she feels that she needs to leave to be happy then you are not going to stand in her way. Read the book, you'll find it helpful.
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If S occurs, what in the world do we/I/she tell our D(5)?
It's tough to explain to kids that young, but the thought you want to drive home more than anything is that it is not her fault that this is happening, and that no matter what happens you will both love her unconditionally and you will both always be there for her. Kids' biggest fears in D are that they did something wrong that contributed to it and that they are going to lose a parent in the process. So focus on those things. And have followups, because they need support throughout the process, not just once at the beginning of it.