I'm sorry you're dealing with this.
I'm not sure it's about wanting to be the "child" in relationships - and I suspect seeing it this way may be counterproductive for all parties - so much as when you've been controlled the majority of your life it is very, very difficult to make sense of being given control of your own life and can feel extremely unsafe. Unlimited freedom in that situation doesn't feel good, it feels confusing and dangerous.
Especially when the control was due to abuse so the boundaries of what was acceptable would change at random without any predictability - which means you don't even have a framework to try and replicate as a starting point for new boundaries and learning to make your own choices within that (as opposed to leaving a rigid institution where it was predictable and you can replicate that to give you a sense of safeness).
It's a combination of simply not being able to recognise that you're allowed to make free choices about xyz now (because when you've never had that you don't notice); not understanding how to make choices and which choices are acceptable vs which choices might "provoke" someone to hurt you; not feeling safe to make choices in case it leads to harm; feeling totally overloaded by the sheer volume of choices you suddenly have to make each day (and navigating these hurdles for each of them) when before you couldn't make any; and just feeling unsafe with having choices and freedom.
I remember feeling like I'd landed on an alien planet and was trying to find my way along a cliff edge in fog with no fencing or lighting, when I left my abusive situation.
I felt so unsafe, was constantly terrified that every decision I made would result in somebody new stepping into the "enforcer" role and hurting me, and didn't have a framework for assessing what I was and was not allowed to do. I didn't have a clue how to work that out.
For example, I used to stand in the supermarket afraid to touch anything because I had no instructions and was petrified if I bought the "wrong" thing that some random person would intervene to attack me. Punishment was what I was used to so I expected it would have to come from somewhere else even if my abuser was gone.
I did lots of things that I look at now as silly, but it was because I didn't spot that actually I could choose to wear a coat if I wanted to or I didn't have to wait until precisely 6pm to take the bins out. My brain couldn't recognise where choices existed because it had never experienced it before. It did not see the opportunity for a choice that other people did.
I spent a LOT of time asking myself (and occasionally others) "am I allowed to... (write a shopping list / wear this / buy this brand of bread / eat 3 slices of toast...).
I suppose I'm sharing this because I wonder if it would feel less frustrating for you to see that maybe it isn't that your mum is trying to adopt a child role, but that she really isn't equipped after decades of abuse to recognise or feel safe with the level of freedom and choice-making most people are?
It does sound like your mum could benefit from therapy with someone who understands abuse and trauma, although of course she needs to feel ready to take that on which is hard. Freedom Programme might be a good stepping stone.
Aside from therapy and connecting with other abuse survivors with similar struggles to me, one of the things that helped me to learn how to spot choices and feel safe with freedom was having safe places to practise (which did mostly come through specialist support). So that was things like regularly being asked for my opinion on how things were being done (e.g. organising a party type situations where there are lots of different options) and then seeing that sometimes that would be listened to and presented back as a choice or acted upon to change the direction of plans. At the same time as the people around me were holding back from telling me what to do or choosing for me. (I recognise I was fortunate with the support I had and am by no means suggesting this is your burden to take on - like I said, most of this was professional support.)
The first and greatest hurdle was probably learning how to even notice I had a choice available to me, from there it was easier to start learning to exercise little choices and build up.
I wonder if that's where your mum still is and whether that might also be feeding her sense that she effectively has to give this man "a second chance" ? Like she doesn't recognise she even has a choice about it, it's not her responsibility and she could decline? To her it's just happening and that's it, no choices to be seen?
If you've made it through all that, I wish you all the best. I would be scared too and I hope you can find a way through.