Cas is an electronic number, and one CAS is for one person. The third understanding in your summary is correct. If you want to go to Leeds University, you should go to Leeds to study languages. Language can be read anywhere. It's not like the college entrance examination. This is not an effective academic system.

Regarding your question 1, once the cas in Liverpool is changed, it really can't be changed, because if the school issues cas, it will have to go to the Immigration Bureau for filing. If you don't go, they will lose a quota. You'd better not change cas in a hurry before you are sure. Although cas can be changed, there are still risks. No one knows what the visa officer thinks, and you can't explain it clearly in person. The main reason is that your time is too tight, otherwise you can wait until Leeds has definite news before changing cas. Personally, in order to ensure the visa to pass, do not change cas.