I did what you said about checking the flex in the middle and it flex’s but so very little I feel like any stiff board will have that semi little tiny flex but I think it won’t effect the pack. As for the fishpaper in each p I’ve also done that. I remember reading on a topic you said to add those fishpaper 
That’s what I did with my 10s4p pack
If you can record a slow motion video with your cell phone while jumping it will give you a very good idea
PLEASE be careful with those balance leads, allowing any of them to cross over will result in rubbing, which will likely lead to a short and a:fire: