Well, I tried the dair with Sonic in training mode but it doesn't change anything, because if he'd die, he can't do it anyway, because of the hitstun.
Air dodging seems useless... yeah I always do it too, because Samuraipanda once said that the CPUs do it and that it helps, but I guess it doesn't.
I tried DI and yes it doesn't help if it's the direction you came from... Oh god I have to stop doing this >.<
Edit 1:
I tried the air dodging in training mode with DK (attacker) and Sonic (air dodger).
Sonic's in the middle of FD and has 73 % so he dies to a DK fsmash (uncharged). I tried it 2 times and he doesn't die if it's 72 %. Anyway, I air dodged even two times with Sonic, but he still dies... only DI can rescue him.
Maybe DI works better if you air dodge... I haven't tried this, but why should it... air dodge goes with your momentum...
Edit 2:
I tried air dodging again with DI but it doesn't help him in any way.
DK fsmash at Sonic in the middle of FD at 90 %. DI up immediately while he get's hit. He survives at 89 %. If I DI up at 90 % he dies. If I DI up AND air dodge he still dies.
Air dodging just seems to help you getting to your normal falling anymation (faster than if you wouldn't air dodge?).
Edit 3: It's not faster^^ air dodging is useless if you want to change momentum...