I can't answer all your questions Dan4G (although I'm sure someone can!) but in answer to the following:
Dan4G wrote:
It appears, by looking at last year's cut off scores and other parents' posts, we may have a chance at Aston/Handsworth. I wonder what would be the course of action in that case
Yes, you appeal for the school that you want.
Dan4G wrote:
I guess, there would be no need to make an appeal? As I understand that the appeal would be needed if DS is not offered a grammar school place.
No, you would need to appeal for the school of your choice. You are not appealing for a grammar school, but for a specific school. People sometimes appeal for a non-grammar when that is their first choice, despite having been offered a grammar. This being because they feel it is a better school for their DC. Best forget about the whole grammar/non grammar in your thinking.
Dan4G wrote:
Or would it be the case that we would need to make an appeal to get a place at CHB if he is offered a place at Aston/Handsworth?
Yes, as above, you appeal for a school.
Dan4G wrote:
In that case, on what basis? Academic (head teacher support, etc)? or other e.g. sibling, catchment? or a mix of both?
Academic, in your situation it is for academic reasons that your DC will not have got a place and so this is the basis of your appeal, non qualification
Dan4G wrote:
Also, what happens if you win an appeal? Would the school take the child from September or when a place gets available in the future?
It depends on timing, you are appealing on the basis of qualification, but if the school is full you will also be appealing on the basis of over subscription. The Q&A Etienne linked above explain this clearly.