Don't get frustrated. There really aren't that many variables to keep track of.
1- I don't know the age of store bought yogurt but I have never had just-purchased yogurt not work as a starter. I have had month old yogurt from my fridge not turn out. If you don't plan on making yogurt often enough to keep you starter viable then consider freezing some in ice cube trays.
Whatever starter you use- make sure it lists "live, active cultures". Personally I wouldn't use a flavored yogurt as a starter.
2- I don't think that is enough starter. I will use 1/4 cup of starter for a quart of milk. If you are using 1 cup of milk that would be one Tbs of starter (12X the amount you used).
3- It depends. The goal is to denature the albumin proteins in the milk. 180F is hot enough if the milk is held at that temperature for at least half an hour. It needs to be held at 190F for closer to 10 minutes.
4- You need to get better control of your temperature. 130F will kill your starter but erratic temperature fluctuations will also produce very poor yogurt. Without a temperature controller or manual intervention your slow cooker will get much too hot- even with a water bath.
More successful approaches are to put it in a draft free place in an insulated container, or in your switched off oven.
5- The humidity is not going to play a role unless it is condensing into your container enough to dilute things.
Good luck. I hope you get it working. Fermenting milk is fun.
I have no idea why it starts with Greek yogurt. Wherever I've had it - at home, restaurant-made, or ready-bought, it contained just plain yogurt, water and salt. (The ratio varies to taste). This includes ayran made in traditional Turkish restaurants run by Turkish owners. Also, I can't think of a practical reason why true (strained) Greek yogurt could make a difference. The whey doesn't have a strong taste on its own, and what little it has, it is similar to the yogurt itself, so you probably won't notice a difference. You are just losing the nutrients from the whey.
In some countries, true Greek yogurt isn't available, and what you get sold as "Greek yogurt" is actually sour cream with double the fat content, but with the same protein and lactose content as plain yogurt. This could have some taste difference in comparison to ayran made with plain yogurt, but in my eyes, it will be a negative change. If you use the same yogurt:water ratio as with plain yogurt, you will get too much fat, which isn't pleasant in ayran (which is mostly a refreshing drink). If you use a doubled water ratio so the fat amount stays the same, you get much less taste.
Conclusion, go with the normal yogurt. It is not only cheaper, it is also the traditional, time-proven way of getting tasty ayran.
Best Answer
Presumably your culture was still doing its work after 8 hours (and cultures derived from old batches can lose potency so what was achieved in 8 hours a few batches ago may take a little longer now). The shut-off time turns off the heat I assume, but the temperature will drop very slowly from that point. It wouldn't surprise me if this batch was actually a little more tasty due to the longer fermentation.
However there could be another cause, especially if you're in the Northern hemisphere. Even if you did everything exactly the same, including the starting temperature of all ingredients, one possible cause would be a change in the milk. I don't know whether you always buy the same milk (and even then the ultimate source can be different from most shops.)
But then it gets more interesting, even if you get your milk from the same herd of cows: milk isn't a uniform product. See for example Seasonal variations in the composition of Holstein cow's milk and temperature-humidity index relationship.. The paper I link (among many others dating back over 100 years) would appear to suggest that both fat and protein concentration will increase as the weather cools off. More protein in particular would mean more to coagulate.