DarlingDuck, I haven't read through all of the posts because DS had me up most of the night with colic so apologies if what I'm about to say has been said but here goes (apologies in advance for the essay!)
My Mum could have written your post a good few years ago. Her & my Dad split when I was 12mo, he soon shacked up with his girlfriend (who is actually love to be fair) & took on her DS who is a month younger than me. For years my Mum made all the effort to get him to see me until I was old enough to call him myself. I would call him on a Sunday morning, he was arrange a time when he was to pick me up, he would always be at least 2hrs late & sometimes he just wouldn't even turn up. Yet, every Sunday I would sit at the window waiting, breaking my Mum's heart.
Of course, my Mum thought he was a complete twat bad Father, but she never bad-mouthed him to me because he was still my Dad. On my 5th Birthday, he came to pick me up to take me out for tea, he was late as usual & I hysterically told my Mum I didn't want to go, he accused her of 'turning me against him' to which she told him he had done that all by himself. I still saw him quite regularly after that but it was always me making contact & his girlfriend facilitating - telling him seeing his DD was more important than playing golf / watching football .etc. When I was 13, my mobile phone rang - I thought it was him ringing to say Happy Easter (I was still hopeful he was going to snap out of being an arse), I answered, he asked who he was talking to before telling me he meant to call his friend, not me. I decided then that this had gone on too long & that I really didn't need him in my life. I have only received one phonecall since then - when I was 16, he called me to gloat say how well my Stepbrother had done in his GCSEs, it was just too bad for him he underestimated me & I had passed double the amount my Stepbrother had to which my Dad replied 'oh, really...? Oh... well done'
Now, we haven't spoken in years, he has no idea I now live 135 miles away, am engaged & have a beautiful DS. His loss, not mine.
My DStepF however, him & my Mum got together when I too was 18mo, he did not adopt me formally but did take me on as his own daughter. Despite him & my Mum now being divorced, he is still, in my eyes, my Dad & my DS's Grandad. He has taken on my Fiance as his Son-in-Law too.
My point is, your DD will soon decide of her own accord that your XP is a dick & that your DP is far better than he could ever hope to be, by all means tell her the truth so she can understand it is not her fault but please don't bad-mouth him, it will only make you look bitter in your DD's eyes if she does choose to try & have contact with him in future.
Hope this helps! :)