Funny thing is the only thing I use my C stick for atm is up tilt, down air, and retreating fair. Considering the rate that I get flat out comboed nowadays I was considering using a c stick to SDI but then I realized I was just flat out DIing wrong in the first place. If I had to choose between down airs and SDI, I'd rather personally take the down airs because the moves you're typically getting flat out comboed by on ike will **** you regardless of SDI due to the fact that a lot of them move as theyre happening (the main one that comes to mind is sheik's ftilt). I nair manually, I even do retreating back airs manually because it doesnt matter which stick you're using, you're better off using A to back air instead of getting an empty or double hop.
As for walk off versus full hop fast fall, I'd rather telegraph something less since telegraphing in of itself is one of ike's worst traits, but I do both as necessary.