Nair into grab hasn't failed me yet, but sometimes an opponent is caught by the tip and it doesn't quite look like I can reach them in time, so I go for Ftilt instead.
Assuming you can't grab combo them anymore, strongest punish on ledge get up attack is Dsmash, capable of killing the earliest. (Ever so slightly stronger than Bair's knockback) Second strongest is Utilt, dealing the same damage but not killing until a good chunk of damage later - but is also 2 frames quicker to come out and locks you in place for much less time in the event that you miss. Ftilt is the least optimal, dealing less damage than the other two and having the same startup as Dsmash but killing around the same time as Utilt.
There may be small differences on some characters' get up attack endlag, though, I'm not entirely certain on that. For most characters Dsmash and Ftilt's 13-frame startup is fast enough to get them.
I'm not sure I fully understand the shield changes, but Ike's Nair and Fair aren't safe enough for him to attack with abandon, so yes, spacing is still crucial. Ike still has a disadvantage on block for those, if I understand the changes right, so shield grabs will ruin his day. But with spacing, he can cover his landing with jab since he reaches farther than a majority of the cast. Against those that can challenge him, though, like other high reach characters or those with tether grabs, it'll probably be a better idea to go into evasive maneuvers instead, or avoid Fairing their shields to begin with. In an Ike ditto Ike can respond to blocking his opponent's Fair by jabbing, but if he tries to grab after a spaced Fair he'll get hit by the opponent's jab instead and going for a slower move could get him blocked and put on the disadvantaged side instead.