I think you have got to stop assuming anything and get the certificates/wills for extra help.
Who are you descended from, or what is your starting point?
Yes, the birth I found for a James Kennedy Buchan was in 1907 in Falkirk, Scotland. If he was in the army/navy/airforce he could have met Florence some time during the war, or could be already in England for whatever reason and not showing because enrolled.
The missing birth for Kenneth Buchan definitely needs investigating - but you do not know that he is definitely related, just probable.
If Florence E Williams is her child, get the certificate for check that she shows as Mother. If Florence E married Mr Payne, get the marriage certificate to check no father showing. As Florence E Payne is dead (?) get certificate to confirm, and why is she still redacted on 1939 - if you think you know where she is, apply to get 1939 opened.
Who is the other Williams in 1939 in the household?
You have put
'I feel Florence E Williams born in 1928, is her illigitimate child, possibly by David W Humphreys as his mmn was Evan. It is likely the Williams and Evans families knew each other as they were marrying.'
Do you mean that you believe the E stands for Evan, or have you seen that somewhere?