1. platform lenght influences maximum train length and maximum waiting capacity (the length of the platform = passengers. so a 320m platform = 320 waiting people max)
2. i believe it only is an aesthetic difference
3. elevated = no
underground = no
but you can make a sunken station by building a road and lowering the middle with the m/n keys and then building a station (http://www.train-fever.com/forums/topic/center-city-10-platform-tunnel-station/)