So, basically I like Ness...
From someone who used to play melee, I have a few questions...
Regarding up-B:
1) for a upward diagonal recovery, do you circle the stick instantly?
2) for a lateral recovery, how do you perform it?
3) is a downward diagonal recovery (think fox in melee, going down towards the ledge) is it possible?
1) Almost, maybe just slightly bigger/slower circle for a higher angle.
2) You want to hit Ness in the dead center of his back. The trajectory of PK Thunder 2 is determined by where Ness' hurtbox is hit by his PK Thunder. That is if you hit the dead center of his hurtbox, which will be around the center of his back, you will go directly horizontal. It doesn't matter which direction the PKT2 is traveling, just where it intersects his hurtbox.
3) Yes, you'll want to do as tight of a circle as possible and hit him in the neck/head which will send him at a downwards angle which can sweet spot the ledge, though it's difficult and can sometimes, seemingly randomly, cause you to bounce off of the ledge and SD.
Other questions:
1) Does Ness throws, make any difference? (front or back)
2) Do Ness players commonly go for Fair > Spike combos?
3) What purpose does N-air serve?
4) Is PK thunder Edgeguard suggested on anyone except fox?
5) Does the back of the yoyo do more knockback than the front?
1) They do the same damage and knockback, so it just depends on which one you've used more recently (staleness) and where you are on the stage.
2) Commonly would be an overstatement but they're certainly possible and very effective.
3) Its hitbox comes out very quickly so it can used to get out of combos when at low damage such as taking Pika's uptilt at 0%. You can DJC multiple nairs together quickly for low percentage combos and shield pressure. It can also combo
into dair in some spots where fair wouldn't have enough knockback.
4) You can use it against Falcon to force him to avoid it by going low thus limiting his recovery options. It also works decently against slowly recovering Kirbies and Jiggs. Obviously it's effective against Ness himself, you can even hit his PKT2 and negate his recovery entirely. Try it on the whole cast and see who it's effective against and in what situations it leaves you open to attack if you miss.
5) Not certain, I assume you're talking about dsmash. It comes out on the back first so it certainly might do more than the "second hit" in the front which is common for many dsmashes. For usmash the front/top is the strong hitbox and the end of it in the back is weaker with less knockback.