If you're trying to be accurate, you wouldn't use "subtle" to mean "deceptive" when speaking of a person. Perhaps it could be said euphemistically though.

Subtle means not being direct or not obvious. You can use vague if someone isn't providing much information or is speaking in general.

It is simply the contrast with "in an obvious way". So that means different things in different situations, but always "not obvious".

It's not very noticeable. Did something change but only slightly? Is somebody trying to tell you to go, by looking over at the door? Is somebody trying to get you to talk, by looking over at you? "Sly" is similar but not really interchangeable. Maybe you are being sly, by misleading someone or sneaking around quietly. For the sentence (He was subtle at the meeting and did not provide clear details) I would say that he did not provide much detail, or that he tended to listen more than speak.

It can be used in both meanings, subtle meaning indirect, undertoned, but also delicate, gentle, yet recognizable. Compare: It was a warm, calm day, with a very subtle breeze. (= calm wind) It took a while for me to fully understand her, she was very subtle about it. (= I finally understood her point, but it wasn't presented directly) As regards negative/positive I think that is to be taken from the context, the word subtle itself having neither positive nor negative connotations, simply describing the method, rather than means.