1) Is the setup ethical? I didn't get any sense of her being unhappy or controlled but you do worry.
I think the Thai operations are generally well run and they look after the SPs making sure they get health care, days off, well fed, cared for and are free to choose what they do and who they see. I think the SPs have openly entered into a business agreement to work as an SP for a fixed period of time in return for a set amount of money. Sometimes these earnings are to help their own lives and sometimes they're working to help family members as their bonds with their family are strong. They're not particularly pressured to work. However, as in all new jobs, sometimes the SP can handle the job and sometimes they don't like it. The ones that don't like it are free to return home.
2) What kind of rotation do they do? How long until she's back?
It varies, but often a Thai SP will return to a place a few times. It depends how long they stay in the country. Many are only in the UK for 12 months.
3) If/when she does come back what's the chances of it actually being the same girl behind the profile?
If the profile has the same photos then it's the same girl. Profiles do get recycled with different names and photos.