As for PP I'm in a strange predicament because I only have a 2DS to practice on and I can PP on 2DS with close to 100% success rate, but since I haven't developed the muscle memory with a gamecube controller my ability to PP kind of evaporates.
Have you had any issues with your analog stick? I stopped trying to Perfect pivot when I realized I was slowly breaking it. Maybe I was just too aggressive? I assumed it was impossible to consistently perfect pivot on the 2DS/3DS version.
Anyway, I agree that Iceballs are so much better than default that it's ridiculous. And the default fireballs are already extremely useful, but they're just outclassed. Iceballs have lower damage, but the lingering hitbox allows for better stage control and you'll end up getting higher damage output when you consider the followups to freezing. (You can get them damn near to freezing percents with a single d-throw combos. Iceball>grab>pummel x 2>d-throw>fair>nair gets them for 37%, and Iceball starts freezing around the mid-40s.) There aren't many followups to default, outside of a grab. Iceball has guaranteed follow-ups and makes Luigi even more dangerous. So B-3 is the only indispensable custom move. It should be run in almost every set, I think. I think we should have some sets with the bouncing fireball because it isn't flat-out outclassed, as it allows for a different kind of stage control. It's not as good of a move, but it's also pretty different, whereas Iceballs come out looking like a straight upgrade to the default.
Mach Cyclone is just inferior to the default. The default cyclone is one of the best down-Bs in the game and the customs just don't compare. Still a good move, but not the incredible move that the default is. We shouldn't have more than a single set with it, if that.
I don't like the Missile customs, although floating missile might be objectively better. There's barely a downside to outweigh the benefits. Personally, I just prefer the default. Probably because I'm so used to it. I hate quick missile. It's actually not that great or safe for recovery, seeing as there is the danger of getting stuck in the stage. It's just not as good as the other two.
Burial Header is good, very good when used effectively, but Luigi loses an incredible punish option and ridiculous KO move. Still, it could have some potential that I'm not seeing.
Edit: Ok, Burial Header is even better than I thought. Been testing out Luigi's customs a lot and I think 3X31 is a critical set for sure. It gives Luigi another great combo-starter. (I still find the default to be the best, but I mostly play against bad players who practically walk into the sweetspot.) Does anyone know if it's as good for recovery? Like, does it go as high or sweetspot the ledge as easily? I can't really tell.
I'm not sure if this has all been finalized but this would be my list:
Critical:
3111
3211
3131
3231
Niche:
3311
3331
2111
2211
2311
3312