DD is multilingual and speaks 3 languages fluently. I am aware there is research to suggest that bilingual children take longer to develop the higher levels of the language until later compared to monolingual children, is this worth mentioning, and does the Ed Psych report go against this as her verbal reasoning score was very high?
I suggest you avoid this argument because of the verbal comprehension score, her level 5 prediction, the very high reading and spelling ages, and her WIAT-II reading and written language results.
Also, will this ability in languages be considered as high academic ability?
To some extent perhaps, depending on the evidence. (If she had already taken GCSE and achieved a high grade in a short period of time, it would have been more compelling.)
Just briefly mention her languages as "background information".
And is it worth mentioning that the grammar school she is applying to has specialist language status and would be better positioned to cater to her needs than the other local alternative that has a very low language take up and whose recent OFSTED report highlights that the teaching of languages is key area for development?
You're getting into specific schools here, and that wouldn't be considered relevant at a Bucks selection appeal where the only issue is whether the child should be deemed qualified.
Finally, the day before the second test paper we had asked DD not to circle question numbers on the answer sheet (she used to do this with questions she found difficult to remind her to go back to them), so she had to develop a different technique to which she was accustomed. The reason we told her this at such a late stage is because it was bought to our attention via the results of a mock 11+ test received in the post the day before the second test. It's not a big thing, but is it worth mentioning? We have confirmation from the LA to say, she circled question numbers on the first paper but not on the second, and we have the comments from the mock 11+ results.
If you've had a letter from the LA about this, you may well find a copy of it in the appeal papers, so you could certainly mention a last-minute suggestion before the second paper. Best not to talk about mock 11+ tests, however ........
With 120, you really don't need much in the way of extenuating circumstances, so don't go out of your way to raise them - or they will start to sound like 'excuses'. The best approach in your case would be simply to answer concisely any questions the panel put to you.
The academic evidence should speak for itself.