Evangeline Lilly said in an interview that she would be appearing in Avengers 4. She said Ant-Man and the Wasp would be like an origin story for her being a hero. So it will at least be connected to Avengers 4.

As for Scott, I think he will be in Infinity War. However, just because he saved the world from Thanos doesn't mean that he, along with the rest of Team Cap, are now free from being criminals. I could totally see Ross arresting Team Cap, or Team Cap willingly surrendering, but Scott shrinks and leaves before being arrested. That would be why Ross is looking for him in Ant-Man and the Wasp.

No. I honestly believe people are way overthinking these dates. July 2018 was the only slot available for an Ant-Man Sequel that would give it enough time to be produced, and not have to move too many of the other films around. That, is probably the only reason it's in-between both Avengers films. I doubt it will be connected any more or less than the other Phase 3 projects.

If you notice Black Panther was originally slated for Ant-Man and The Wasp's current release date. But was pushed back after the latter was announced. If the movie in-between Avengers: Infinity War and the Untitled Sequel was meant to largely tie in, Marvel would not have done this. Just food for the thought.