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.
You are correct that the milk is heated to denature the albumin so that it becomes part of the structure of the yogurt instead of washing out in the whey. When distributed through the yogurt properly this protein will not cause the clumping problems you are seeing.
You shouldn't expect to make ricotta from yogurt whey- even if the milk wasn't boiled it just doesn't work well.
Most yogurt problems, including breaking and clumping, are caused by poor temperature control. Heating the milk too much during incubation, over incubating, or erratic temperatures, can all cause your bacteria to misbehave. Often this causes the yogurt to be too acidic and to curdle which would explain your clumping.
As has been canvassed in other answers; the best yogurt incubation temperatures are between 100 - 110°F (38-48°C) but it seems to vary a little with the starter. The best results seem to be had from putting 110°F (48°C) milk in an insulated container to incubate rather than trying to use a heater.
Many yogurt recipes call for powdered milk to boost the milk protein in the mix. Another possible explanation is if you mixed it in insufficiently.
Best Answer
The point of heating the milk to 180F (and then holding it there for a few minutes) is to denature some of the milk proteins so they will participate in the yogurt structure and improve the texture.
You wrote that you added milk powder. This is a good thing because it adds extra protein. It is perfectly reasonable for there to be enough protein that simply heating it will form a loose curd. Acidifying the milk would produce a more firm structure for yogurt or cheese, etc.
Milk can also curdle prematurely if it has accidentally acidified already. For example if it has started to spoil from a mixture of wild bacteria. If your milk is of questionable freshness then I would recommend throwing it out to be safe.
If you are confident that your milk is fresh and has been handled well, and if the curd is fragile and tastes like milk, I would recommend inoculating it and fermenting it for yogurt as normal.