AGree with Kalni - I think it's "too soon". My advice would be to not invite him at all, but certainly go. Ask anyone else to help BUT him. Expensive or tricky is way better and will get you farther along the DB path - consider it an investement in yourself!~He has to feel the full effect of his actions and that means YOU doing a little walking away from him. OTherwise, it looks like you are condoning his behavior.
I know it's hard and I'm not saying I could have done it - but in no way would I invite him to ANYTHING. Not for months from now. He left, he needs to be left alone. It's also pursuey. YOu need to prove to him and yourself you are independant!~ Believe me it will go a looooong way.
The main point is YOU need to know you can cope without him. It may not be perfect, buy you will do it and BELIEVE ME on the other side YOU WILL FEEL STRONGER. You might not stay as long as you like, or have the perfect childcare solution, or project 100% to your peers, but you will gain confidence knowing you moved on with your life - WITHOUT H.
Trust what Kalni and I are saying - especially Kalni. She's very very experienced and wise. I'm pretty far along as well and have learned from my mistakes as well as my successes. every time I moved toward independence, I was shaking in my boots and sobbing, but on the other end I found myself. This will help us either to move on if the time comes, or to challenge H to stand by his decision to leave or not.
OF course if you end up divorced there will be lots of coparenting opportunities that will arise in many shapes and forms. But now you are DBing and that means being independent. That means letting him go to dig his own hole. And as you are not able to get much space since you're coparenting all the time at home, it would actually be really good for you and your R to have that space. You would also see that you CAN do things without him. That will increase your self confidence as a LBS.
That said, if you are going to send the letter, I'd DEFINTITELY send the second one~! Short and simple is the motto when dealing with childcare arrangements with WAH's. ALWAYS.
