I would leave the class teachers to do the undermining! Just try to be as factual as possible:
Unfortunately the head made "specific reference to a small behaviour incident that occurred in Y5 and which has never recurred. His behaviour record is usually exemplary. The HT also did not highlight his exceptional ability in Science. The class teacher had not seen the reference. Having seen it, his CT has now written a glowing reference. His CT from his previous school has also written a glowing reference."
With regard to the extenuating circumstances, why would they affect his 11+ and not his excellent SATs predictions? Or has there been a marked improvement in his school work this year?
Allocated school has inadequate science provision.
I suggest: "The grammar school has better facilities than the school allocated, and would cater so much better for my son's interest in Science."
Does the grammar school do separate sciences, and the allocated school combined science? If so, it's an argument you could use.
I assume this will be a foundation school appeals panel, and I'm afraid that - on average - they tend to be tougher than LA panels.