Your point 5 is inaccurate. Page 12 of the Admissions Code does not suggest that foster siblings should be treated as full siblings. It says that where there is a sibling priority (and there is no requirement for there to be one), the term 'sibling' must be clearly defined, and that it should be clear if foster siblings are counted as full siblings.
Your point 4 is debatable. I haven't seen the Police letters, but if they do make it clear that there is a continuing threat from pupils in school A, that the OP has been advised to take specific action to avoid that threat, and that the only practical way of achieving that is for the two boys to go to the same school, then I think that a) the allocation of school A is approaching the threshold of Wedensbury unreasonableness (that the decision is so absurd that no rational admitting authority would make the same decision in the same circumstances), and that as school B is the nearest school, and it resolves the transport issue for both DNephew and DS, and it removes the threat from pupils at school A, that it is reasonable to hold that school B is the only suitable school in these circumstances.
Logistical issues are rarely good grounds for appeal, but this situation is one where resolving the logistical issue meets the educational and social needs of the child.
This is all with the huge caveat that I'm assuming the letters from agencies are clear and direct. If they are, in fact, "I understand Mum has concerns about DS travelling to school alone because of a past incident", then that weakens things considerably.