The whole point seems to be arguing that ONLY the school under appeal is suited to your child’s needs
I agree this is a good way to try and approach it, although, technically, no, you are just arguing why you want a place at this school.
if the panel have seen a detailed conflicting argument for another school, surely the appeal is hugely undermined.
I share your concern, but try not to despair. If the panel act very 'professionally', they will not allow themselves to be influenced adversely by what they have read. Indeed, most panel members are experienced enough to realise that parents will often be putting forward very different cases for different schools, and they may well sympathise with the position you have been put in to the point that they go out of their way to treat your case with all the care and consideration they can muster!
For a previous instance of a panel member clearly acting with great professionalism, follow the link contained within this post:http://www.elevenplusexams.co.uk/forum/11plus/viewtopic.php?f=35&t=19289&p=230842&hilit=dejavu#p230842
Another possibility is that they will not even have started preparing for the appeals yet, and that the appeals team will get to them before they do begin. Many panel members I have known preferred to read the papers 1-3 days before the appeal, so that everything was fresh in their mind. Believe me, with so much paperwork to get through, they would be mightily relieved to get a message that something is to be discarded, and will not want to spend unnecessary time reading it!
Is this something that the ombudsman might consider maladministration or is it just a blow we’ll have to live with?
If your appeal were to be unsuccessful, I think you would have grounds for asking the ombudsman what he thinks.