Here's what I intend to do - while it's not the voice of experience it's a position I feel I can live with long into the future. Maybe it will give you some insight into your decision.
I wish my wife happiness in whatever she does. I will respect her and remain her friend. But I will draw firm boundaries if she wants to remain married or become separated.
I will not be a doormat and move out of my house, away from my children, so her best friend can move in rent-free. If she wishes to separate for time to think and be alone - fine. But that shall be her action and her choice. She will not bully me out of the house.
If she wishes to start seeing other people - fine. But that shall be her action and her choice. I will express that I don't believe these actions belong in a marriage or separation. If will tell her that if her intent for the separation is simply to date others and try out new relationships then it is nothing more than a facade and she should consider that and just move on. stuck808 is right - using someone as a "backup" shows little respect for him/her, the marriage, and the family involved.
I don't have the answer on the divorce thing - my situation and position may indeed lead to it. However I will make my words known. I will be firm and fair. While these might not be the words she wants to hear now, I will have the respect of myself, my family, and maybe one day even her.
peace, fairness, kindness
"My actions are my only true belongings. I cannot escape the consequences of my actions. My actions are the ground upon which I stand." Thich Nhat Hanh