I assume you are talking about them SDIing the dair in the low % pillar combos, like on CF? I think if you space it correctly you can guarantee you land the shine, but the only way I've been able to do that is to barely hit the front of their character so that if the SDI in front I will have my momentum still be going that way and I won't miss, and if they SDI behind then the back end of my shine still hits (Shine is much more disjointed on Falco's back than front). Unfortunately, unless this a punish on lag, it usually feels too risky because it is horrible spacing if they WD back or just dash dance (since you usually want to overshoot aerials to cover those options). So if you overshoot your aerial and they SDI behind you can utilt (harder to hit and kind of risky, but a higher reward) or do a quick autocancel bair.
As far as nair vs. dair, I think my tendency is to approach with dair when I am more confident it will hit. Nair feels like it reaches further, and it comes out faster, so it's usually easier for me to space. When I dair it's usually when I know they can't avoid it because they are stuck near the ledge and that doubles as an advantage for if it hits that it's much easier to dair -> dair spike someone than to nair and try to edge guard at the ledge height which always seems awkward.
That's my take on those questions, but I'm interested to hear what other players think because this is mostly stuff I've just learned to do instinctively; I never really put thought into them when deciding what would be good.
And yeah, like Van said, just overshoot those aerials. I would just get grabbed constantly for that **** until I realized I should just run a little bit further before SHFFLing. If you DO **** up spacing, jab usually covers you pretty well. It is at least better then spot dodging immediately to avoid the grab (something else I used to do all the time lol).
