Olá, @bldotan. By default, the word ainda means still or yet, like you said. For example:
Ainda tenho o livro = I still have the book
Ainda não sei = I still don’t know = I don’t know yet
Other uses of this word are idiomatic variations which don’t add much in terms of meaning. It’s more of an emphasis/reinforcement kind of thing. This is the case for your 1st and 3rd examples, where the meanings you attributed to the word ainda are actually already implied in the respective sentences anyway:
- Ainda hei de ver esse filme = Hei de ver esse filme → Even without ainda, the sentence already says that at some point in the future (one day…), you’ll watch that movie.
- Ainda há pouco lá fui = Há pouco fui lá → Even without ainda, the sentence already says that a short while ago (= just), you went there. Note that the preferred word order is a bit different when the word ainda is omitted. Also, keep in mind that the word só could never be used here, because só = only, and here we are using just in the sense of recently.
So, it’s usually very clear when you need to use ainda or when you can just add it for flavor (a very idiomatic one)