2. If Airwave shows that it maxes out at 40%, plenty of times it bursts to over 60% and then drops after that. Streaming only increases it more.
I see that the Apple TV's phy is a-HT-20-1ss, which means that its top association rate is 72 megs. If you have 10 to 15 users on that access point that are even further away, the AppleTV is going to be significantly impacted when it tries to stream anything, because those users farther away on the other side of a concrete wall are probaby going to drag its performance down. Long story short, you need to figure out how to get that AppleTV on 5ghz if it can support it, and fewer devices on that access point, otherwise it will be hard to maintain a reliable stream on it.
When you say CCI is not an issue, at 10% retries and 15 other users, CCI is an issue. If you do a packet capture on that channel while the AppleTV is streaming, you will be able to see what is connecting to it and causing your problem. Unless you figure out a way to confine the RF from that AP to be only in the space where the AppleTV is, it will continue to compete with other users on the side of the wall.
It is easy for me to say based on the limited information you give me what it is, but you might have someone take a look at your deployment to see what can be done about your issue.