I think you need to really put things into their respective buckets and see some perspectives. I also did not like how others label the cheater as the rapist already. That is a tad too harsh, since they do not know his behaviour in general. But one thing stands out a lot though. You mentioned that you guys are in a small town, and I dare say that most people here are from different cities, and maybe some are in the UK too. People's values differ. But one true thing remains though. No trust in a relationship means no relationship. That's it.
To me, it goes to show that he cannot have those boundaries set into place if he is in the same town that he grew up in, and knows everybody else. Is that why he went ahead and harassed someone else ? What kind of town is this ? In most big cities, people would actually hold each other in an arm's length kind of way, and draw a respectable boundary and line. He hasn't done this.
Having read the whole thread. I do also agree that distance is your ally here. Even if you do not feel like you can let him go emotionally and end the relationship asap. (I have been there myself.) Distance or separation will indeed be good for both of you. To let your anger, and to let your emotions come out slowly. Without throwing these angers or sadness against one another. So separation is to protect both yourselves. See it that way.
Yes, you may have a lot of ideas now about why things were not the best, but that should not excuse him of his actions though. You need to write them down and remind yourself.
The part about being a new father is true, and the people in my life I see their relationship have stabilised over a long period first before they have their first child. Maybe this intrinsic trust happens first. (You know, you get your every day bickerings out of the way first and find a workable solution and know how one another ticks, and learn to be there for one another.) But even so, it does not excuse his drug taking, or his burnt out step. He still need to deal with this. You also need to not get involved in this personal process of his either.
Future-wise, how can he be a father if he cannot take care of himself, and make better choices which doesn't risk his life, and therefore not able to look after his own child? Has he actually realised his responsibility in the far future? Has he even talked about kids, and how fatherhood affects him? Maybe he has not woken up to this awareness. If you guys are like < 30, I can see why this has happened. You need to see it in perspective and get an absolute good grip on this. But don't force and push yourself though. Cos you will resent yourself so much if you do. You still need to align, your heart, with your head, and with the action that you decided to do. Even if he feels bad, and he did things to please you but it is still against his own heart, and he doesn't say things and reflect on it. It will also end up in tears.
I just wish you luck.