Saying that something won't hurt us - and then have it ACTUALLY HAPPEN and find out we were completely wrong - is EXACTLY what happens.

I told my husband to date.
Hell I begged him to date so he'd leave me alone.
One weekend some things went down at his parents house involving SD17 who has lived with them since she was 5.
He was living with them and unbeknownst to anyone, I was well on my way OUT of MLC. Anyway, he took off by himself for the weekend and the kids and I didn't know it. I was right at the edge of going to him and when that weekend came that I didn't know where he was, my mind went wild. I was hurled out of the tunnel for good that weekend. The thought that did it? That he was with another woman. That I could never go back. I'd lost my chance. When he finally called, I totally unraveled.
How I must have sounded crying and trying to get in 2-3 years worth of explaining and apologizing.

My point is you don't know how your dating might affect her.
It sounds like you are almost gloating to BIL.
And don't try to say you didn't know he might tell her, either. You knew.
And you WANTED a reaction.
Well, if it was her that hung up on you, it's because she DOES care. But she isn't strong enough to reach out yet.

RMG, you have one foot planted in the world and you're dipping your other foot in the church occasionally.
Your choices have been made crystal clear to you through this. Stand or don't.
If you are not standing for your marriage anymore, that's certainly your choice and everyone understands but I am left with one question that I want you to really think about and just answer to yourself.

Why have you chosen not to move to Surviving yet?

I think it's because a part of you still wants to stand but you just can't face the lonliness and pain - and the feeling like a fool at times for doing it.

But that's just my opinion.