I'd like to add that IMO shotmaking talent will overcome the lack of a lot of specialized knowledge. But here are my answers to your questions.
1. The level where you are a threat to beat them or their protègè. I'd say A level and upwards?
2. Yes, there are people that are willing to help, but they cannot always do so. There are also people who will hold out on you. I have a friend for instance, who is brilliant at a certain shot that everyone is bad at. I've been hounding him for years for his "secret", and I know he would never intentionally hold out on me. He just can't explain what he's doing. I've also asked a great player for his advice on a certain shot that he does very well, and he flat out told me that he won't teach that to anyone. Yes, I did offer him money for the lesson. It's ok, I totally get it. It would be way worse if he did what someone else might, actually lie about what he's doing.
3. Agreed. I'd also like to add that what people are actually doing may be different from what they think they are doing, which is why I added the "watch more, listen less". Sometimes, though, it's hard to see what they do because they are so quick.