1. I think they are just long-lived or just like Dragons are otherwise their parents would still be around.
2. No i dont think so.
3. Yes but they are probably dead from old age
4, Genetics of ponies are very difficult to explain and doesn't really make sense (although its a cartoon so i guess it doesn't really have to make sense) I think its is possible to have an Alicorn foal but the odds of having one are extremely slim.......Say if a Unicorn and a Pegasus mated then the odds of having a Alicorn are like 1% versus the other odds (Earth Pony (due to recessive genes,yeah i know its boring biology, 33% chance of the foal being a Pegasus and 33% Chance of it being a Unicorn)
5. No comment....I assume Alicorns can change it from their Natural hair to the glowy and wavy hair.
6.Maybe it was an animation error