Ok so if we straighten this out your dd is 11. She started secondary school in september.
She was friends with anna and Bettie at primary school. When she went to secondary school she started being controlled by Lisa. She also made friends with Mary.
Mary was also friends with Lisa from before starting secondary. Because of being controlled by Lisa yoir DD stopped hanging around with Anna and Bettie.
You found out DD was being manipulated by DD. So rather than stopping contact with Lisa decided to get them all to be friends and introduced Lisa and Mary to Anna and Bettie at a play date with your daughter.
NOW all the girls have decided to gang up on DD and bully her based on your illness. Thinking you had CV and so therefore your DD too. This was done by phone and text and online.
In the bulling DD and Mary were together and Lisa, Anna and Bettie were together (why are they not social distancing unless you are not in the Uk).
All the girls were meant to go to Annas house but when you told her mum she cancelled (you thought this was punishment for the bullying.)
However they really just changed the meet up venue to Starbucks and invited Mary (who was probably already invited to the house meet up so what's the difference?) You thought Mary would stand up for your DD and not go. So now your DD has no support. If these girls are as nasty as you say Mary probably did not want to get on the wrong side of them.
However you should remember Mary was friends with Lisa first so she may not be the best person to support DD or have the confidence to stand up to Lisa, Anna and Bettie.
You should definitely tell the school about the bullying. You should also report it to 101 the non emergency police number so she has a crime reference number. Bullying is after all a crime. You can inform the school that you are doing this and ask them to tell the parents. Say to the school you would like apologies to your DD from the girls and for them to stop. Otherwise you will take it further with the police.
Do not engage with the parents or children anymore. It obviously just causes you both more stress. Also keep all the screen shots but block these girls from contacting your DD in future. Also try and get your DD to look for some other friends within her class.