Hi. Using Markers from a geojson file is possibly easy (depending upon its size...can you provide an example?). Using an existing kml file without converting in to a geojson file is difficult using Blocks. It is much easy to build a map with Markers using a csv file (see below).
The Paris Tour Guide tutorial is an easy way to work with Google Map tiles. If you are having difficulty with this tutorial you probably will have lots of difficulty building your app to import kml or geojson files. The least difficult way to build your app will involve you using the MIT Map component that works with OpenStreetMap tiles . The Map component is documented here> Map components
There is no single tutorial showing how to do all the things you described. An extensive list of Forum discussions that contain example code that you can use for parts of your Project are here > Search results for 'maps markers' - MIT App Inventor Community .
An easy way to create a map from a List of Markers (not a kml or geojson) is shown in the National Parks tutorial. The tutorial demonstrates how to plot multiple run time markers using a csv file to an OpenStreetMap. Use the list from csv Block to convert the csv to a List. What you do depends on how you make your List, where it is stored or is downloaded from. That tutorial might be a good starting point to learn about using the App Inventor Map creation tools.
A good way to get directions from one place to a Marker is described in HOW TO: Use the Navigate control
After you learn to use the tools, you should be able to build all parts of your Projecct using App Inventor.
Regards,
Steve