Hi, saskia. I'm sorry things have continued to be difficult with your mother. I am hoping that being sectioned is going to get her the medication review and care that she needs, to get her more stable and to keep her safe.
I've not been in your exact situation and of course nobody can predict quite how it will go. However, I would agree with your brother and others here and advise you to stay away and let the professionals get on with it. As much as it likely sounds hurtful to read this, it's possible she will be calmer/more compliant without family there (I know this is true of my mother). I don't mean to suggest that you do anything wrong, just that the nature of dementia and family relationships means that often the people closest to the PWD, get the worst of it (again, been there and done that).
My mother underwent the American version of sectioning, and it was the best thing that could have happened to her. I know not everyone has a positive experience and that your system is very different to ours, but she finally got a medication review, got really thoroughly medically checked out, got treatment for her physical issues, and really improved in the time she was there. Just eating nutritious food regularly (she was underweight and malnourished), and sleeping all night, and being clean, and getting the medications she needed (and not whatever she was over-and-underdosing herself with at home), and not wandering and being anxious all the time, made a huge difference to her well being.
I can only hope this has a positive outcome for your mother and for you.
Yes, I'd pack comfortable clothes, and I'd mark them if you have a chance (I use a fabric laundry marker, like a Sharpie made for fabric, and mark the tags and/or the fabric itself). You can also take a quick "inventory" of what you're packing for her, by taking a photo or two with your phone. This works better for me than making a list or thinking I will remember what I've packed! (My mother is currently in rehab/respite after a hospital stay.)
And I would definitely take her a couple of books, an author you know she likes or even something she has been "reading" recently. My mother has always been a big reader and even though I know she can't really read anymore (she can read the words, but not follow the narrative or remember anything), it's comforting for her to have a book. So I would definitely pack one and then take more in for her later. I pick up used copies of books/authors I know my mother likes (because the ones in the care home go missing/walkabout a lot, and even more so at the rehab place) for cheap at used book stores, charity shops, library book sales, and the like. Even online from A----n sometimes!
In the rehab/respite facility where my mother is, and in hospital before that, we have a small (5 x 7 inch) framed copy of a photo my mother has in her care home, and had at her home before that. It's a nice shot of my husband and me dressed up for a cousin's wedding. I got a color copy made and bought an inexpensive frame and put sticky labels on the back, that not only say it belongs to my mother, but also my name, my husband's name, and our relationship to her. I don't know how much my mother likes it (I often find it in a drawer) but many staff in both hospital and rehab have commented that they like it, as they "recognized" us immediately from the photo. Plus it gives them something to make conversation about, with her. I don't mean to add things to your to-do list, just that it's an idea.
Please look after yourself, Saskia, and update us if and when you feel up to it. We're here to listen. Sending all possible best wishes.