AirPlay Quality

No. Typical iTunes 16/44.1 ALAC will be streamed intact as 16/44.1 ALAC to an aTV2; however, it will be output to an AVR at 48kHz. That's because the aTV2 outputs all audio as 48kHz, which is why you can AirPlay a video to the Apple TV with DD 5.1 and have it properly detected by your AVR.

It's up to you to determine if that ALAC 44.1 to 48 conversion (going on within the aTV2) affects the final quality of the sound. I do not think it does, but there are some folks who prefer to listen to their music through the optical out of a Mac or an Airport Express, which would pass 16/44.1 ALAC unchanged to an AVR.

yeah, moving from a Mini to the Apple TV does simplify things in many respects, though the tradeoff is that means your Macbook has to be in the house, with iTunes running, in order for the aTV2 to find its library. It probably also means a little tighter integration within the Apple walled garden, and more Handbraking of video to the aTV2 preset.

There'd be some value for you to 1) also load music onto an iOS device because you can just as easily AirPlay to your aTV2 from an iPad et al as you could from a Macbook, 2) you may discover you enjoy controlling your music listening experience from an iPad or other iOS device, which you can carry around the house with you, more so than the Macbook, and 3) it's still a good idea to keep an inexpensive second Mac around, something 4-5 years old but that could step in and function as your iTunes media server at home in the event something happens to your Macbook.

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.

You could also get iTunes Match and stream it from the web, but it's only 256kbps as I'm sure you know. I wonder if they will ever offer lossless (or if they do and I don't know about it)?