i haven't messed with it all that much but i'm sure there's an answer to that
well, let's try theory-fighting for the sake of discovering something
on normal jump, you're in the air for 39F. add 4F startup and you have a 43F duration that you're in the jump. assuming the opponent doesn't tech, the game allows 8F of leniency for emergency tech, and then 18F that hinders everything, resulting in 26F of "forced knockdown." if they forward roll, they are invincible from frames 1~3 but can be hit out from frames 3~17. let's use the last frame of the forward roll where you can get hit out as reference.
supposed you jump right when they hit the ground and the 26F of forced knockdown starts. the moment they are able to do something, you're left when 17F of your jump. at this point, you won't be able to use anything to punish the forward roll, but you can still hit him while they're guarding for something.
this is why you don't jump right when they tech, and instead do it earlier than the moment they hit the ground. i'm guessing if you jump 8~15 frames before they land, you can punish them with something. i know in my video that i jump right when they tech/hit the ground but i'm going to say right now that you shouldn't follow me at that point
try experimenting with jumping right after 3C hits and see what results you get
neutral tech is 32F in duration, so if you jump at the time they tech, you have 11F before you land, in which you can be hit out of. even less reason to do the jump right on their tech.
EDIT: as for the reason you use j.B over j.C, i would prefer using it because it has more active frames. therefore it is easier to meaty with j.B than with j.C, even though the damage off of it isn't as good.