Hi Scoliosis warrior
Welcome to the forum.
You might want to have a look at the following FAQ which explains where everything is;
www.benefitsandwork.co.uk/guides-for-claimants/faq/forum .
Your questions are outside the remit of the forum.
We cannot answer your question as we do not have all the facts such as, how old is your son.
Your CT may be affected when your daughter turns 18 as you would loose 25% single person deduction, so that would not make any difference if your son move back home. We would advise you to contact your local Welfare Rights Organisation:
advicelocal.uk where they can all your circumstances into consideration.
Gary