1) Large Import and Troubleshooting should shed some light, a lot depends on how many locations you try adding to one map, Google MAPS API functions and the time to draw a map will have a big impact and every time a map tile is drawn it will hit your limits and will bill you accordingly.
2) Depends on your server, your configuration, the WP CrOn limits etc. again look at the Large import t for more info
3) I do not know, I have never tested 100K locations on my site as the bill from Google would be astronomical to load and geocode I cannot test for you
Geocoding is responsible for 35% of the Google billing fees with the Google Maps API on most accounts. Also you may be getting throttled by Google when loading that many locations to geocode Unfortunately there is not much you can do to eliminate the map rendering fees.