Personally Im not sure if they put too much thought into the opening... because if it was masters of the arts wouldn't the Fire be the Fire Lord instead of his daughter? She is gifted but not as powerfull as him. When I first heard the idea that they were his teachers I really liked that but the chances of that seem to be slim because of Toph. Who knows maybe the intentions for those people changed as the show progressed and the Earth Bender was suppose to be his master but ended up becoming an unknown guy (at least I dont recognize him). But I dont think there is any way, at least right now, to clear this up for sure. ^^;

No, the designers are not that random. Most seasons are already planed in advanced so they knew what charactors they were going to create. The thing that conserns me is that if the earth bender in the start is the strongest earth bender in the world, then that means he/she/it would be stronger than Bumi or Toph. Now we will wait to see if it is friend or foe.

16 more days untill the fury of the avatar.

Could the beginning earthbender be Xin Fu? He is a strong earthbender

Or Master Yu