I wasn't trying to worry you any more than you obviously and understandably are, but it's a classic ploy by the bullying partner (predominantly the male) to want to have her back - aafter all, as far as he's concerned, he's in charge of her, she's simply his so called chattel, and all his false sweet talk will play on her emotions to take him back, especially when he swears blind that he'll never do it again? (Yeah, right!)
At only 21, she is of course and adult in law, but she's still really a child, certainly in your eyes as a concerned parent, and presumably has no sort of experience of such an abusive relationship (hopefully?).
Your situation, i.e. very worried mum caught in the middle, is extremely delicate to say the least, because you obviously want what's best for your daughter, but at the same time you need to let her spread her wings into adulthood.
Without having read all the posts, it would seem from what you've said that because she's taken him back, has the Police involvement come to nothing? You can only speak to her, without him being present, express your concerns, which I'm sure you've already done?, and then let her get on with it. Like anyone else, she will have to learn from her mistakes, for we all make them.
It concerns me that this guy, you say, is "her new boyfriend"? I wonder how much she knew about him before co-habiting with him? Perhaps he has "previous" for assaulting his partner, something which maybe your daughter didn't know about? Any guy who can assault his partner to the extent of leaving her with lots of bruising, cuts, scratches etc, doesn't sound to me as if he has much, if any, respect for her.
I hope she comes to no further harm and makes the correct decisions for her own wellbeing and peace of mind, and yours?