Well, here's the initial chart I made for the characters nair and spring locks.
Remember, this chart displays the maximum percentage the character can be at while taking a fresh spring drop or sour spot nair in order to become locked on the ground. The colors are as follows:
1.
Light Purple = MAY be nair locked from a sideB
This means that it is possible to nair lock these characters from a sideB footstool, but it's incredibly situational, mostly relying on the terrain of the map, and whether or not the opponent immediately reacts from the footstool. If they move the instant they hit the ground, the nair setup WILL NOT work.
2.
Red = Setups into jab lock will almost never work
This means that the spring AND nair lock setups will fail. It only applies to Luigi, and only because he slides WAY too far to start off a jab lock before he goes out of range. If he stays on the level, however, you CAN run up and grab him or hit him during the forced-get up from the spring or nair lock. Luigi can be considered a Light Purple category when it comes to the nair lock.
3.
Green = SideB > footstool > nair jab lock setup works
On these characters, sideB directly into a footstool, followed by a fast fall nair, will work at least 80% of the time, the only exception being Bowser, who's probably the most difficult character to set this up in, despite his size. Samus and Lucario both surprisingly share a weaker, but similar effect to why it's so god**** easy to lock ROB. They tilt back when footstooled, especially Samus, so you're actually closer to the ground when you footstool them, allowing you to nair into the jab lock. Lucario is probably the second most difficult only because he isn't fat, so the nair is hard to land. Olimar, of course, would be third since you HAVE to aim for the helmet. Mr. Game and Watch and Jigglypuff are in the same boat as Olimar in that they're so small that they fall longer to hit the ground, but not long enough to break out of the tumbling animation. Game and Watch's animation when hitting the ground is so long and creates such a good sized hitbox that landing a nair is no problem, while Jigglypuff is so wide that even with her DIing in any direction after the footstool, the nair should hit.
So the order of difficulty in initiating sideb > footstool > nair > jab lock would be the following (from easiest to hardest):
-ROB
-Samus
-Jigglypuff
-Mr. Game and Watch
-Olimar/Lucario
-Bowser
4.
Blue = sideB > footstool > nair lock won't work
Simple as it's said. Squirtle and Ice Climbers suffer from the Luigi Effect, where they just slide wa~y too far for the nair lock to be reasonably possible. It may be doable vs. Squirtle, but if so, I suppose the difficulty must be insane. However, because they don't slide TOO far, spring into jab locks still work, but are slightly difficult to pull off.
***One thing I forgot to mention. All characters CAN be nair locked of course, but the ones not colored can ONLY be nair locked if they are hit with the sour spot after they hit the floor from a tumbling animation. One of the best ways I can think of to get those uncolored characters into those animations would be to hit them off a platform while they shield, then follow with the nair.
Then again, jab works much more reliably. Whatever floats your boats.***