Saskia, I'm so, so, so sorry to read your account of how the sectioning and initial visit have gone. I can't even imagine what you must be going through.
I have very little advice to offer, but a couple of ideas.
I am sure it would be fine if you talked to the unit and let them know she likes porridge for breakfast. In fact, you could even prepare an "About Me" type of letter or document and give it to them: favourite foods, her routine, some background information, whatever you think might be helpful. My mother's care home had official forms for this, but when my mother went into rehab recently, I just typed up a short version and emailed it to them.
When my mother went through the US version of sectioning, I was asked not to visit her that first day, but given a phone number to reach the nurses' station. I called and was given a detailed update, and reassured I could call anytime, day or night, for information or if I was worried. On the days I didn't visit I did call for updates and they were reassuring.
I did the same thing when she moved to her care home, as I didn't visit her for a few weeks. It was not only comforting to hear she was okay, but also a good way to get to know the staff, especially as I wasn't going in.
I hear you on the jewelry fears. My mother was also still wearing her jewelry when she went into hospital. I got lucky that one of the lovely nurses was able to coax her to remove her jewelry and put it in the hospital safe, "for safekeeping," and then I was able to take it home with me. She only ever asked about it once (despite having worn one of the necklaces for my entire life and the other for at least 15 years). So I wonder if you could have a word with the staff and perhaps they can tell her "it's hospital policy" or coax the ring off her with some excuse? It can't hurt to ask.
And my first visit to my mother in the secure unit of the hospital where she was sectioned, was beyond words horrible. (I'll spare you the details but in her heard, it was all my fault and she was going to let me know about it.) The visits got better after that. When my mother went into hospital she wasn't eating or sleeping or taking her meds properly, so just taking care of those issues helped her a lot, and then they were able to get tests and a diagnosis and get her medications sorted. I completely understand this may not be your experience, but I do hope for your sake, that it gets better.
I also think that it is okay not to visit, if that is what you need to do.
Going by my experience, and what I've read here on TP and heard in my support groups, everything you are thinking and feeling is to be expected in the situation. Not that it makes you feel any better, I'm sure.
My husband spent a LOT of time telling me (and so now I am telling you):
-your mother is safe
-she is getting the care she needs
-you didn't do anything wrong
-you are not a bad person
-you are not a bad daughter
-you are making sure she is being looked after
-it's not your fault; it's the dementia's fault
-YOU ARE NOT A BAD PERSON
Please, saskia, please try to be kind to yourself and remember that you did nothing wrong and you are not the enemy here. The dementia is the enemy: hate it, not yourself. It is so, so easy to say that, and so, so difficult to do it.
Sending much care and concern your way. If you feel inclined, please update us when you can. We are thinking of you.