Hi there. Happy flying!
1. Yes. You can create a flight as you would like. You can even set this created flight as the default flight, this will replace the Cessna/Migs flight. In the "create flight" window just enter your parameters such as departure airport, destination airport, the type of aircraft you want to fly. Then simply save it and check the little box labled "set as default flight". It will load that created flight everytime in place of the Cessna/Migs field flight.
2. Not that I am aware of. But of course if your flying low level, you want to do that on a clear day to where you can see clearly outside the airplane to avoid those pesky 600ft high hills. ATC isnt very good at determining ASL or MSL levels anywhere in the entire sim. So its a good idea to keep an eye out for obstacles. Reminds me of the cartoon jingle "Watch out for that....(boom)..TREE!"

3. The files you are looking for are in fact text files, but have the file extension of .cfg. Do a "file find" for "aircraft.cfg" and "airport.cfg", the files will come up and you can add what you need in a regular text editor like notpad.
Hope this helps.
ps...watch out for that tree...err hill!
