In regard to the questions, I'm generally treating the knowledge question as a Unit 1 question, and the sources as a Unit 2 part B question. The main difference is that I mention historiography in both questions, and don't analyse the provenance as much as perhaps in the Unit 2.
Firstly, the a (knowledge) question is indeed similar to a Unit 1 question. Although I am sure you have already had a look at the relevant markscheme, it may be worth reminding you of the difference in emphasis.
For Level 4 (19-24 out of 30) the mark scheme is the same as Unit 1 with the addition of;
with some evaluation of argument.
As you rightly say, this is where the historiography comes in. For Level 5 (25-30 out of 30) they expect two differences from the AS Unit;
'A sustained analysis.. (rather than an analytical response) [and] '.. evaluating arguments and - as appropriate - interpretations.'
So, very similar to Unit 1 but more so, if you see what I mean. There is less allowance for loss of focus into detail or narrative.
With regard to the structure, I would suggest that you can't go far wrong with the following;
Introduction: clearly setting out argument i.e. whether you agree or disagree with the question and, ideally, how far.
Yes I do agree
No I don't agree
Conclusion: coming to a judgement as to how far.
I don't think this is telling you much you don't already know. However, one final reminder to have a look at the Examiner's Reports for examples of previous candidates' work.