I’m sure someone more experienced will chime in, but in my limited experience so far, you can assign a due date with a tag to one particular line, regardless of the date of the note, but it won’t show up in your calendar. None of your notes show up in your calendar unless you actually link them to an event (or create a new event).
I have been using some due date tags for things that are not actually calendar events. For example “#due(20180604)” turns into “due(Jun 4, 2018).” There are probably quicker ways to type the date; I use military format and it translates. There doesn’t seem to be any way to view these due dates later in an organized way, but I go to “Search All” and enter “#due” and it pulls up all notes that have any due date. They seem to come up in order of project rather than chronological order. I think it would be most helpful to see them chronologically.
As for showing only tasks, I think it has been discussed a few times in other posts, so the developers are aware folks want this feature.