You will get the class arrangement for the upper years. The LA will have to include that in their case for the appeal. I suspect they may be snowed under preparing for appeals at the moment and therefore aren't in a hurry to send you information that they have to include in their case.
Off the top of my head, I would expect 5 or 6 classes in juniors. I think 5 is more likely with an average of 32 children in each if the school is full. If they have 6 classes the average class size would be 27. Either way it is unlikely to be significant for your case.
I find it it surprising that "no-one has ever won appeals for this school in the past". These will not be infant class size appeals so you don't have to show that the LA has made a mistake (although it still helps if they have). On average around 35% of appeals are successful. For a school with a PAN of 40 to have never lost an appeal would be unusual.
You can certainly mention the new building but the panel may be nervous about basing decisions on a building which isn't finished yet.
It would be worth finding out the calculated net capacity. It sounds like the agreed net capacity is 280 and that they are currently full. The calculated net capacity is expressed as a range and might be, for example, 280-310. If they have set the agreed net capacity at the bottom of the calculated range you should bring this up as it indicates that the school can take more children without undue problems.
You should definitely ask whether having an extra child in year 1 has caused any problems. It is quite likely that the answer will be no, or that any problems relate more to his SEN than the fact that he is an additional child. If they've had an extra child in Reception and Year 1 and the sky hasn't fallen in, that helps to convince the panel that the school may be able to handle one or two additional children.
I don't think there is anything in the Admissions Code that is particularly helpful here. That mainly comes into play where the LA is doing something wrong, which they don't seem to be on the information you've posted.
The main problem you've got is the number of appeals. Whilst it isn't impossible, it is unlikely that you will persuade the panel that the school can take 12 more children. The panel will therefore be looking at whether the prejudice to your child from not being admitted outweighs the prejudice to the school from admitting your child. Make sure you have the best case possible for stage 2.