Naxxramas definitively won't come out in 2016. We are discussing possibility to test Naxxramas on PTR, however each test would have its own part - for example tests per quarters, tests per boss only and so on.
Of course we will let you know if something like this is going to happen or not.
As for the Ahn'Qiraj, I'm pretty satisfied although it was not and still is not perfect. We learnt from the mistakes and we will try to avoid them in release of Naxxramas and future releases on Kronos II.
In the meanwhile, once session persistence is live on both realms, I'm going to focus on currently available content on Kronos I, postfixing remaining issues in Ahn'Qiraj, Zul'gurub, Blackwing Lair and even in Molten Core, with secondary focus on transport system (Mac clients RIP, pets on transports, ...) and generic mechanics (pets, tweaking mob AI, classes).
Erminn, PTR of course would reduce the amount of bugs introduced on live realm, however few of them appeared only after several runs over the raids. Sadly, you cannot be sure enough to have it bug-free. These things will always happen, in lesser or greater format.
The release of Ahn'Qiraj was pretty tiresome for me personally, as the entire release was upon the players, Ahn'Qiraj opened its gates around 5:30 AM server time (my time as well) if I remember correctly.
During the release I have not slept for almost 48 hours due to final preparations. Also the pre-event took really big time to develop, which I'm pretty sad about since its a one time event, worth nothing but visuals.
However, I believe, we had the best opening event (beside "blizzlike" lag in Kalimdor) ever any free Classic server had. I'm hoping that you satisfied at least a little bit, too.
Naxxramas will come out in its complete glory and shine.