1) That’s strange, assuming a good WiFi connection, it should take 10-15 mins for that size download. Pocket Earth v2.2 will be out in about a week and has some improvements to maps management as well as being more robust. Hope it will solve this issue!
2) GPS can function well completely on its own without WiFi or Cellular. It will show you your precise location on the map and you can even use the Route Navigation mode (Play button on route info panel). However, Airplane mode must not be on as this disables GPS as well. Instead, you should disable Cellular/3g data directly in the Settings app, as well as WiFi/Bluetooth (if you want to save battery). Final caveat, without Cellular, the initial location fix can sometimes take a bit longer, like even up to 1 minute in worse cases, but once it locks on, it updates continuously.