Since the first part of your question was answered in the general thread, I'll answer the part on the air D moves.
8D is primarily used as a combo tool to extend air combos, while 2D and 6D are generally used in pressure. 2D can be used on an opponent on the ground to start pressure or to catch someone mashing an anti-air (it should work at around max range in most matchups). You can also use 2D while airdashing to change up the speed and angle of your dash afterwards. It can make for some interesting pressure or a situational cross-up. 6D is mainly used in air-to-air situations. For a simple example, if you throw out a 214A from its max range and your opponent jumps after blocking and you jumped too, you can use 6D as Ignis is returning to you to keep them blocking.