Power Shielding the Jab
Okay, I did some more testing with the Jab, and who can Power Shield it. I could not come up with anything that made sense as to why certain characters could and others couldn't. Character weight, height, and falling speed don't seem to relate to who can and who can't PS the jab. (Not counting the bottom section, which is clearly height.) So I'll just list which characters fall under the different sections.
When testing, I found that there were four areas of the jab. They aren't necessarily different hitboxes, except for the tip, but they all get different results. Here's a picture of what I found.
Note: This is not an exact replication of where the different "hit boxes" are, it is just to give you a general idea of where they are.
1. The area furthest from snake (the "tip") results in the character being popped into the air by the first two hits, and it is impossible for any character to PS the last hit.
2. The area 2nd furthest away from snake (red) is an area where all characters can avoid the third jab because (a) they get knocked too far away by the 1st or 2nd hit for remaining jabs to connect, or (b) they have more time before the third hit hits them and they are able to PS the final hit. Note: This area is really small and hard to hit with, so I wouldn't worry too much about this.
3. The next area (yellow) is where the normal jab is. This area is what the following chart applies to, because this is the only section where there is variation depending on which character you are fighting.
4. The last area (green) is when the character is pretty much directly on top of snake. This "hit box" is almost exactly the same as the yellow one, and characters are rarely ever this close to snake for it to really make a difference. The only thing that changes is that some characters who are able to PS in the yellow section are not able to PS the last hit. But since the chances of getting a character that close to you in an actual match and how small the "hit box" actually is, I didn't bother including it in here.
Characters that Cannot Power Shield your Jab
Captain Falcon
Diddy
G&W
Ganondorf
Ics
Jigglypuff
DDD
Kirby
MK
Peach
Pit
Ivysaur
ROB
Samus
Wolf
Yoshi
ZSS
Characters that Can Power Shield your Jab
Bowser
DK
Falco
Fox
Luigi
Pikachu
Snake
Sonic
Toon Link
Ike ***
Link ***
Lucario ***
Mario ***
Marth ***
Charizard ***
Sheik ***
Wario ***
Zelda ***
*** = They can only power shield if they are DI-ing away from snake. This is difficult to do if the person is not expecting the jab and is ready to DI back.
Characters that aren't even hit with the complete jab
Lucas
Ness
Olimar
Squirtle
Unless these characters get "tipped," or they are right on top of snake (the two green "hit boxes"), or they are DI-ing into snake, they get hit too far away to be hit with the remaining jabs.
Note: All characters can Smash DI out of the 1st and/or 2nd hits to avoid being hit by the third jab.