You say you weren't going to throw away 30 (?) years of marriage because of one awful evening. I think that's a very good call because if it's correct that this incident was a one-off and a huge shock to you all, then I'd like to say that you shouldn't be overreacting a moment longer to the behaviour of your Daughters or their boyfriends. Your DH has apologised, he's filled with shame and remorse, he's tried to get a medical assessment of what happened, he's making promises about future behaviour. And you are filled with shame and sadness and desperate for some sort of forgiveness from your children.
But, do you seriously think that your Daughters' reaction is fair? I don't think it is and they seem hell bent on wringing the most out of this situation and making it all about them. Why? How does this help their Mum or resolve the fallout from an unpleasant incident on holiday? It should be concerning for all of you that you can say, when she's now in her 30's, that your eldest Daughter has always resented your DH because he left you when she was 2?!! Two? And. you have been happily back together and had another child since then, but she is always sparring with him and is so 'headstrong' that however hard he tries to help her always, she just doesn't have an easy relationship with him. Well, set your DH's outburst aside because she's clearly a very angry person and, wherever this has come from, is determined to make him feel bad about the past - a past which she would hardly remember and which should have been resolved when you re-married and were all together again.Him trying to 'make up for' a bad decision he made a long time ago and when she was tiny, is surely going to be exploited for all it's worth if she feels she can manipulate him now.
They were suggesting that you leave him when this happened? And how does his threatening you when he is clearly suffering some sort of reaction to wine, pills, heat, dehydration, compare with her then actually punching him in the eye? It appears that she owes apologies for her own behaviour and is hardly in a place in which she can sit and judge the behaviour of her own parent.
I would strongly advise that you leave things as they are. You won't want to admit that you have been treated badly but both of you have - irrespective of what happened, some understanding and maturity is well overdue and both of them should be reassuring you about what happened, not ignoring or further distressing you, or telling you how to live your lives. They're not small children, they sound rather spoilt and entitled and now need to step up and not try to make both yourself and their Father feel any worse. If they have anger issues - and the eldest does sound as if she has a problem - she needs to consider getting some help for herself.
I hope that you'll stop trying so hard and give things time to settle. You have nothing to punish yourselves for and I'm sorry if this post sounds a bit harsh but you sound like good people, so please don't allow your family to make this into something it's not.