Crem, from everything you've said about your DD, I don't think she'll respond well to being treated as younger than her age. Did you say she's 20? Maybe not but if she's 18 or over, I'd treat her as an adult & support her choices, even if they're (from your perspective) utterly wrong. That will maximise the chances that she'll turn to you (and still be speaking to you) if it all goes wrong.
I know your history with her is especially difficult. I find it hard enough that DD no longer wants my help with things & is keen to make decisions without me - and DD has had very little upset, or even unwelcome change, in her life.
In your position I imagine I'd want to be more than averagely protective but I'm not sure that would be good for my DD (might be, for someone else's) and I'd still need her co-operation & agreement to put it into effect, otherwise life would be one long battle.
It's no longer up to you to make everything all right for her. If only, but it's not. Hope you manage to find a way through and I think QQ's 'kiling with kindness' is a good start.
MI, glad you spent some time on your birthday with someone who knows how to celebrate. You remind me I need to start hinting now, to avoid another fiasco like last year.
Rose, well done for feeding and supporting the bereaved and I'm sorry for your loss.
BD, is your boy 12? (Sorry, having trouble with ages tonight). At that age DD lost everything all the time - coats (many) travelcards (several) school bag ( more than once). We didn't make her pay, we got her to help with retracing steps, phoning Lost Property etc. She really was trying to look after her stuff and just made mistakes through distraction etc. She has hugely improved and is probably better than me now, certainly a lot better than DH 