I've posted on this subject a few times so, at the risk of repeating myself, I just want to say that my late Husband was sectioned 5 times in 8 1/2 years. Each time his care was exemplary and his was observed and treated with the greatest of love. The thought of our loved one being in a MHU is often so terrible to bear but your Mum will get the help she needs-I'm glad to read that you are feeling a bit reassured that the personal care is going a bit better.
With regards to looking for a different home for your Mum; Pete's SW wasn't up to much and he sent me to look at totally inappropriate CH's-too few staff for residents ratio, so I took it upon myself to find the correct home for him. It really does depend on how good you think the SW is.
Anongirl, this probably feels as though your worst nightmare has come true; but your Mum needs to be observed, and possibly her meds 'tweaked', before she is stable enough to go to another home. In the MHU she will receive both physical and mental health expertise.
Take care
Love
Lyn T XX