Correct: you cannot prevent it from importing your old posts currently.

The idea of importing as drafts has come up before, so that should be pretty easy to allow. You could also probably write a pretty simple filter/plugin that would do that for you (the importers allow you to filter things relatively easily before they’re imported/created).

Importing new stuff only seems reasonable; just starting from “now” when you set things up.

The maps and tweets you see on this site are all powered by the data that’s imported using the importer plugins, but the actual presentation and rendering and whatnot are controlled by this theme (Homeroom, which I also wrote). There are plugins that will leverage the geo data (as an example) which the importers store for each post where it’s available, but out of the box you won’t get any maps or any visualization.