I am a British Citizen and my Thai wife has been in the UK with me for 2.5 years. We are now filling out the FLR(M) form to apply for her “leave to remain”. We are including her Thai daughter (my step-daughter) in the same application as a dependent child.
She was 17 years old when she arrived in the UK with her mother and will be 20 at the time of the FLR (M) application. After studying for two years she is now working. She is still single and living with us and relies on our support so I believe she is still eligible for a visa under this category. I have the following questions:

Question (1) Regarding the step-daughter, what documents are best to give for the following evidence requested in section "13C Child of a Person with Limited Leave as a Partner of a Settled Person in the UK"?

1. Evidence of where you normally live.
2. Evidence that your parent plays an active role in your upbringing.
3. Evidence that you are not living an independent life.

The form says that this evidence should be dated within the last three months which makes it quite difficult.

Question (2) There seems to be a duplication of the evidence request for the “dependent child applying with you” in sections “13A Children” and 13C. Does anyone understand what is the difference in these two sections and why there seems to be a duplication?

Question (3) Section 13C “Evidence of your sponsor’s parental responsibility for you”. What is required and who is this sponsor? Myself being the sponsor of my wife, or is my wife my step-daughter’s sponsor? I can only think of supplying my wife's Por Khor 14 document again. If I am the sponsor then I don't think I have any document showing parental responsibility for her.

Question (4) I have two British children who obviously don’t need to apply for a UK visa. In section 13A it asks for evidence for all children about where they normally live and that you and/or any partner play an active role in their upbringing. They ask for this to be dated within the last 3 months and I'm therefore struggling with this as we haven't had any recent letters. I was going to ask for a letter from the GP and the children’s school and nursery school. However, our doctor's surgery always seems reluctant to do this kind of thing and I’m not sure they'll be keen to write 3 letters for the 3 children. Or maybe they can just list the 3 children on one letter. Any other ideas?

Question (5) My wife also has a Thai son who is now over 18 and lives in Thailand and will never come to the UK. He is still a dependent at the moment but I'm not sure if I need to list him in "section 4 dependent children" as he is over 18 and doesn't need a visa. If I don't need to include him it will save me from having to get one more birth certificate translation. Any ideas?

Any help would be much appreciated. Thanks.