I think, when things have broken down between parents, it is too easy to see the other parent as the cause of all frustration and the ideal target for any niggles. If you had not had children you would have gone your seperate ways and got on with your lives end of story. Having a child means you cannot do that and all those frustrations you get at the end of a relationship that you would previously have whinged out to a girlfriend/your mother or seen off with a really good night out are magnified and made more difficult to deal with. Added to which you have to deal with losing control of a certain portion of your childs life.
It is never easy and it is the easiest thing in the world for me to preach at you with allthe right answers (all of which you know anyway), however, it is the hardest thing in the world to put it all into practise. FOr me that key point was when I realised how good and in control I felt the day he tried to start a row and I decided not to take part (I failed the next few times though!). What I realised was that, to engage with him I had to care about him in some way - or still be tied to him soemhow. So, I made the concious decision not to care, I vented here and to friends I bit a hole in my tounge on several occasions and counted more or less to infinity and back! Very quickly he realised he was not going to get a rise outof me (and his immediate reaction was that I must be screwing someone - cue drama and outbursts) eventually though he gave in and it became easier.
He still iritates me almost every time he goes through my head as he is forever doing idiotic things but now I just grumble to myself, shrug my shoulders and move on. It takes time and effort, but it is possible.
As for not showing up Scramble, if your children are old enough tell them the truth, Daddy got tickets for a show, I guess it must have been sold out other nights - your children will learn the truth eventually and it is better that they do not see you defending him. However it is equally important that you are not seen to condem him either. If they are not old enough then just tell them he cannot come and that you will call him to try to arrange another time - be neutral about him, it will serve you in the long run when they realise Daddy is an idiot and Mummy is kind and fair!