Official comment
From https://support.dronesmadeeasy.com/hc/en-us/articles/211429163-Opening-a-KML-from-Email
"define a polygon, line string, or set of pushpins in Google Earth and import it into Map Pilot as you normally would (via Email or AirDrop) but rename the file from sample.kml to sample_boundary.kml to import as boundaries and linear flight plans. Adding the _boundary to the filename tells the system to import is as boundary definitions instead of just guidelines. The imported data will show up in the File Manager screen. All features that are being imported need to have a name associated with them."
Linear plans will be as shown in the line string. Polygons will show up as boundaries.
You do not need to be connected to the aircraft to plan. You need to have the correct aircraft selected in the settings. You don't need to set camera settings like exposure at the time of planning since they don't affect the layout.
When offline you need to double tap where you plan on taking off to get the lines to show.