Hi op, I think you've had a really hard time here and have taken it in the chin despite a lot of it being unfair.
The thing that first jumped out to me from your posts is that you seem guilty, maybe I'm wrong here but I think you are wanting to make things right to ease your guilt, that's not a criticism by the way it's just how it comes across.
You cannot change the past. Ds is here. He has two loving parents and a step dad.
What you can do is look to the future and remind yourself that you have been quite fair with ex.
You say he's from a nice family, has many good attributes etc but you knew he wasn't right for you and vice versa, it's bloody hard to walk away in your own and ok you got back with other ex and subsequently married him but you did not just stay with ds's dad because you were pregnant, I agree with that.
Here's my experience of co-parenting-
Was with ds's dad for 7 years in total. We were really young when we got together but bought a house, got engaged and tried for a baby, I went through a lot to get pregnant and ds was very much planned.
When he was born my exdp became very jealous of the baby getting the attention, missed his carefree life and became a nightmare.
We had a nice life on paper, I was thoroughly miserable and left with nothing apart from ds and our clothes when he was 2.
We had several years of arguments, nastiness, name calling etc and one day we actually talked and decided enough was enough.
We have since shared parties, school plays, holy communion, parents evening etc and life is so much better for us all, especially ds.
He's now 14, his dad is married and has a 2 year old. I have a partner and a 6 year old dd. We all get on, many people find it strange but we don't, after those years of hating each other we have both realised that we are both his parents and will have many future events to attend as mum and dad.
Your ex will possibly be hurt still and if anyone would have told me all those years ago that we would be friends I would have laughed my head off, but we are. I hope the same happens for you.