I've sweetspotted on every stage from almost every angle. It's definitely possible, no real tips I can think of, you just gotta practice up. Probably one of, if not the hardest recovery to sweet spot in the game.
This. Took me a ****load of practice to be able to consistently sweetspot my recoveries, especially from below.
I can offer advice though, a lot of it.
As soon as you activate PKT, you have to judge whether or not your opponent is going to either come offstage or stay onstage to edgeguard your recovery (such as, Fox players like to shine spike so I try to activate PKT just shy of where Fox can run off and shine with his aerial speed considered). PKT2 goes pretty damn far, so you can bypass a number of offstage ledgeguarding tactics if you're patient. Also, don't forget you can Magnet Stall offstage if you need a bit more time to assess the situation before starting your PKT recovery. Don't get predictable with that though, always account for "how deep" your opponent can go to safely stop you from recovering (that is, how far out they can go to edgeguard you while still being able to recover themselves; if they go beyond that then they are usually just trying to style).
But back to the point. If you're looking to sweetspot from below, it can be kinda hard to consistently get it if you start PKT just below the ledge. That is, if you're looking to sweetspot by hitting yourself with PKT within the first second of starting the move, you're not going to be sweetspotting from below unless you are either quite some distance below the ledge or digging your face into the stage at an angle (as on FD and other examples you provided). But there's a simple solution to this: Stall the PKT2.
PKT is out for quite some time in PM; a little bit longer than in Brawl, but I didn't really notice how much longer for my first couple of weeks playing PM. Once I did find out, I didn't realize its true significance until I was messing around in training mode one day and got to practicing my recoveries again. I found out something brilliant. Ness can actually stall his PKT2 for about 3 seconds in midair. To give some perspective, if you start your PKT at ledge level, stalling it to the fullest will bring you back up to the ledge with a sweetspot if you perform it correctly.
To stall it: When you start PKT, curve it down in either direction as soon as you can (the one that corresponds with the direction you want to launch yourself in obviously), and guide it straight down. Before the PKT expires, guide it into yourself by curving it back up. It'll take a split second or two for it to hit you, depending on how much you stalled it, but it'll shoot you up at a near-purely vertical angle. If you start the PKT on the side adjacent to where you want to go, you can send yourself in that direction by making a bit of a wider turn when you guide it up into yourself. It can be tough to get the hang of, but it does pay off (you generally won't have to do this because the sweetspot allows you to grab the ledge if it's behind Ness anyway; it just helps if your opponent is looking to ledgehog you or you start PKT earlier than you were anticipating and want to try landing on the stage).
If the opponent is really close to the ledge when you start it from just below, it's usually not a bad idea to just PKT2 asap (while minding your angle to minimize the possibility/severity of your punishment). But if you're just beyond the ledge while below stage level, and they're charging a smash attack or something, if you don't think you can make it over them your best bet is to go for a sweetspot. If you can infer when they're going to release the smash attack though, just getting that near-perfectly vertical angle by stalling a bit really helps, and can throw your opponent off, effectively lessening or even eliminating your punishment. Doing this does leave you in special fall, so be careful and consider whether it's safer to fall over your opponent or just let yourself fall helplessly for a bit/retreat to the ledge. Remember that you can also fastfall during your special fall as well, so make use of that where you see it fit.
Hopefully this helps, I've had a lot of practice with it since recovering with Ness in PM used to be one of my biggest qualms when it came to using him. Now I feel a bit of excitement rather than dread when I have to play my recoveries. Still, if you have any other questions about it, feel free to ask.
