Getting Nair to combo and getting the opponent to land in one direction consistently will be a challenge. Here are some factors we should take into consideration.
I suspect that certain hitboxes in the Nair are giving more frame advantage than others. Maybe because they simply lock them into stun longer, or they spend more time stunned in the air and the stun wears off once they hjt the ground (unlikely but whatever.) Or perhaps the hitstun is uniform and the trick is to land while leaving your opponent as high as possible in the air.
We should take into account our movement direction and speed. While you're dragging the opponent with you in nair its possible they'll end up being dragged behind you as you move in a certain direction. We should test out if our mobility, the direction we're facing, and how fast we are moving has any influence on how much frame advantage we get. Fastfalling should be either neglected or kept constant seeing as that's another variable we need to test out on its own.
As far as fastfalling goes, I've personally gotten different results with regards to where the opponent lands depending on the timing of my fastfall, but I'm not too sure about frame advantage. I'm pretty sure there's about 3, maybe four different fastfall timings in relation to how many hits of the nair you'll end up getting in. The timings aren't too hard to get right consistently either, to the point where they'll be second nature with some practice.
Both of the above should be tested first independently and then in conjunction so that we get as accurate info as possible on how to make this combo work. Also, I'm sure you all know this, but characters react to nair in different ways, I know for example fox ends up in front of me much more often than shiek did. Depending on the factor that determines whether or not you can combo off Nair, it might be easier to combo some characters over others. There's also the possibility that we're not comboing out of hitstun always, but sometimes the narrow window of regular landing lag. Which might be improbable, but might account for some of the discrepancies with the training mode combo counter, since I'm pretty sure it doesn't count landing lag as a window for extending combos. If it does turn out to be that, then we will have some characters that will be objectively easier to combo since we'd have a bigger window depending on their soft/hard landing lag. I dont know which one the game forces them into, can someone clarify for me?
I'm mewtwo.
I'd do some of this myself right now but it's late/early. Will hit that lab later on.