I guess the more I think about it, the more I'm torn between simplifying or keeping the mini.
Well, I could make the case that "simplifying" and "keeping the mini" aren't mutually exclusive: you keep the aTV2 connected to your TV and AVR, keep interacting with it via the iPad, let it become your primary (or only) playback device, and redeploy the mini (headless even) to act as an iTunes home sharing media server. That frees the Macbook up to do what it does best--be a portable workstation--and your life in the house will be simpler in the sense that you're not managing a computer connected to the HDTV, there's very little tinkering and tweaking involved with the aTV2, wire both the Mini and the aTV2 up to your network, and you'll have a very stable system that stays in place 24/7. You choose whether you let either or both devices sleep, if you do in my experience the wake on demand feature works well.
Any Intel Mini would do quite well as an iTunes media server.
I should share that I'm not actually taking this approach myself, I have both an aTV2 and a 2011 mini hooked up in the living room and plan to keep it that way, I'm just saying I can sense why you're considering doing this--and see the benefits in that approach.