I hope we don't have to do custody fights over our child (I hope we never get an actual D) so I don't have a whole lot of knowledge on the topic, but it does seem that you need to have EXTREMELY clear boundaries with her. She seems to have one set of expectations for you and one for herself. I don't know what your custody arrangment is, but I know how violated they can easily become (having come from a D family myself).
It seems pretty clear that she doesn't respect boundaries per that argument a few days ago when your S wanted to show W his digging work. Even if you were a few seconds late (as it would seem), the fact that she doesn't seem to understand the inappropriateness of making her unhappiness known in front of your child is pretty telling. And then for you to have a really good reason as to the tardiness (I mean, what child doesn't want to show his mommy his hard work? Especially at 6yo), and her to still hammer on is a little off-putting. Granted, maybe she had a really hard day, but it's inappropriate for her to do that to you in front of the child.
I would just be very careful about where your boundaries are with W in regards to the kiddo. Given how she acted at the playground, I think you handled yourself well. And I think writing out a nasty letter and not sending it is brilliant too. Keep up the hard work!
Hi dueinMay -- we're not in a custody situation right now. We do have an unofficial parenting plan in place, and my wife and I have been flexible about time spent with our son. My frustration wasn't due to the tardiness, but her disrespect of me. Sure, she's resentful and frustrated, or might've had a bad day, but the point is, I wish she could see how childish and -- from our Christian perspective -- how worldly she's behaving.
Where's that divine slap-in-the-face when she totally needs one right now? Heck, even I do need one right now, because I'm definitely no role model of unconditional love myself either.