I do not agree with faith or Rocked...but I will leave them up.
And unless I missed something, her not coming home was BEFORE she said she gave him up and decided she wanted to start dating you.
Your AS IF IS a delicate thing. The very thing you ASSUME you can often create. Unless you have a new reason not to trust her (if you mean by driving by you saw his car there), then trust her. If you saw his car there, keep your eyes and ears open. It could have been a goodbye, he could have been stopping by unwelcome. Keep open, but don't jump to conclusions.
I do agree with City Girl: I would not write a letter, text, call or e-mail anything to your W. Honestly, the conversation need not be hard or long. Any decent family therapist would tell you that setting your boundaries is a 2 min. conversation and your W need not "think" about it. It's yes or no. If she is unable to provide you with the necessary safeguards to keep your marriage affair free and do so with remorse and openness you will know right away.
Plus - if you handle this right YOUR demeanor matters. Body language, eye contact and tone are essential. By setting YOUR boundaries and NOT delivering ultimatums you are leaving the choice up to her to make with free will.
so--why not practice your word choice here.
"I would love to date you, but because of our past I can only date you exclusively when you are ready to provide (emails cell phone bill....) as proof, because of ______." Or whatever it is you need. Try on a few scenarios.
I don't agree completely with this: Don't emasculate yourself by using the word delicate or any other "touch feely" verbiage. Firm, loving action is required and that will require a masculine approach. You don't have the power to push her either way (none of us do). Set your boundaries and let her decide freely.
You have not emasculated yourself using the word 'delicate'. It was an appropriate use here. And every relationship has a different atmosphere to it. Use what has worked before.