I noticed that marble doesn't consume ~300gb of my hard drive's space (about the space needed for all of the OSM data), so is it fetching the open street map from the internet? Is there some way to specify that you have the osm data downloaded so it doesn't do that if that's that's going on.

Packet sniffer confirms that OSM data is retrieved from the web. I'll dig around in the source for my other question, but I'll let the other question remain just in case someone can answer me before I discover it: is there a way to specify that there is local OSM data downloaded that should be used instead of retrieving it from the web?

What kind of data format are you having? If you donwloaded .png tiles, you can use them.
We want to do a GSOC project this year (see here) to help integrating .osm.xml/.osm.pbf vector data from OpenStreetMap. Currently you can open .osm.xml files manually from within Marble, but it's not possible yet to automate it in a map theme (or load large files).